What was the inventory turnover rate for Fortinet in 2020-Q4
call there is a new attack service or new ads need to be covered. So that's where especially we working with a lot of service provider for the 5G services to a lot of enterprise and connect all these different devices in the OT IoT space. So that we do see is a huge opportunity and so with our position, with the carrier service provider and we do see the 5G can be one of the driving -- growth driver factor for us this year and it could be material toward end of the year. Yeah, is that going forward is also -- it's a huge, huge opportunity even secure whether -- yeah, the whole -- it's part of the whole infrastructure and switch grow moreover fast and a lot of our carrier service providers starting to have a investment in this area also.
Keith Bachman -- BMO Capital Markets -- Analyst
Okay, interesting. Okay. And then Keith one for you. For the guidance of '21, you talked about capex. Any other puts and takes you want us to think about as it relates to OCF or operating cash flow?
Keith Jensen -- Chief Financial Officer
No, not really. I mean, I made the point about inventory, the turns came in for us pretty strong in the fourth quarter, but I think that's a direct reflection of the success that we have in the product revenue line in the fourth quarter. So that was probably better than we expected. I do think during this pandemic era that we'll continue to maintain a somewhat higher levels of inventory. I think that's in our best interest. This extended payment term program, I think that every CFO wants to wanting that down as fast as possible and every distributor wants to hold on to it for dealer life. So that will be an ongoing battle for us throughout 2021 I think.
Keith Bachman -- BMO Capital Markets -- Analyst
Okay. Well, congratulations to the whole team. Good set of results. Thank you.
Keith Jensen -- Chief Financial Officer
Thank you.
Operator
Thank you. Our next question comes from the line of Ben Bollin with Cleveland Research. Your line is now open.
Ben Bollin -- Cleveland Research -- Analyst
Good afternoon, Ken, Keith, Peter. Thanks for taking the question. My first question, you've made your aspirations pretty clear in SD-WAN, could you share with us a little bit about aspirations, intentions as you move into SASE in zero-trust, how you see yourself positioned?
Ken Xie -- Founder, Chairman of the Board, and Chief Executive Officer
SD-WAN is a part of the SASE offering. What we do is a little bit different than competitor we build within the FortiGate, FortiOS, which also can be offer whether it based on the physical appliance or the virtue software or kind of cloud delivering and that's where the new FortiOS 7.0 give all this flexibility and connect a lot of other part of infrastructure and security service together. So that's where we'll continue to see SD-WAN and keeping growing probably -- by market started probably worth keeping grow like 30%, 40% year-over-year this year.
We do believe we're also keeping gaining market share and at the same time, the 5G is another opportunity come out. We already offered in the new FortiOS 7.0, which also could be a pretty good drive for the additional growth that we have.
Ben Bollin -- Cleveland Research -- Analyst
Could you also talk a little bit about how you envision FortiOS 7 rolling out once available, how backward compatible will it be for legacy appliances? And if you've looked at some of the historical OS refreshes, how long does it take the footprint to roll over as this rolls through the base? Thank you.
Ken Xie -- Founder, Chairman of the Board, and Chief Executive Officer
It's really depend on customer. Some I have to say the channel probably react little bit faster and then there is enterprise then the service provider, sometimes take a little time because some service provider they also have to support in some of that, but we do see this [Indecipherable] a lot of new opportunity. And they also like these that tightly integrate approach whether SD-WAN or SASE which goes security driven networking which through are able then to offer additional service, addi
call there is a new attack service or new ads need to be covered. So that's where especially we working with a lot of service provider for the 5G services to a lot of enterprise and connect all these different devices in the OT IoT space. So that we do see is a huge opportunity and so with our position, with the carrier service provider and we do see the 5G can be one of the driving -- growth driver factor for us this year and it could be material toward end of the year. Yeah, is that going forward is also -- it's a huge, huge opportunity even secure whether -- yeah, the whole -- it's part of the whole infrastructure and switch grow moreover fast and a lot of our carrier service providers starting to have a investment in this area also.
Keith Bachman -- BMO Capital Markets -- Analyst
Okay, interesting. Okay. And then Keith one for you. For the guidance of '21, you talked about capex. Any other puts and takes you want us to think about as it relates to OCF or operating cash flow?
Keith Jensen -- Chief Financial Officer
No, not really. I mean, I made the point about inventory, the turns came in for us pretty strong in the fourth quarter, but I think that's a direct reflection of the success that we have in the product revenue line in the fourth quarter. So that was probably better than we expected. I do think during this pandemic era that we'll continue to maintain a somewhat higher levels of inventory. I think that's in our best interest. This extended payment term program, I think that every CFO wants to wanting that down as fast as possible and every distributor wants to hold on to it for dealer life. So that will be an ongoing battle for us throughout 2021 I think.
Keith Bachman -- BMO Capital Markets -- Analyst
Okay. Well, congratulations to the whole team. Good set of results. Thank you.
Keith Jensen -- Chief Financial Officer
Thank you.
Operator
Thank you. Our next question comes from the line of Ben Bollin with Cleveland Research. Your line is now open.
Ben Bollin -- Cleveland Research -- Analyst
Good afternoon, Ken, Keith, Peter. Thanks for taking the question. My first question, you've made your aspirations pretty clear in SD-WAN, could you share with us a little bit about aspirations, intentions as you move into SASE in zero-trust, how you see yourself positioned?
 Ken Xie -- Founder, Chairman of the Board, and Chief Executive Officer
SD-WAN is a part of the SASE offering. What we do is a little bit different than competitor we build within the FortiGate, FortiOS, which also can be offer whether it based on the physical appliance or the virtue software or kind of cloud delivering and that's where the new FortiOS 7.0 give all this flexibility and connect a lot of other part of infrastructure and security service together. So that's where we'll continue to see SD-WAN and keeping growing probably -- by market started probably worth keeping grow like 30%, 40% year-over-year this year.
We do believe we're also keeping gaining market share and at the same time, the 5G is another opportunity come out. We already offered in the new FortiOS 7.0, which also could be a pretty good drive for the additional growth that we have.
Ben Bollin -- Cleveland Research -- Analyst
Could you also talk a little bit about how you envision FortiOS 7 rolling out once available, how backward compatible will it be for legacy appliances? And if you've looked at some of the historical OS refreshes, how long does it take the footprint to roll over as this rolls through the base? Thank you.
Ken Xie -- Founder, Chairman of the Board, and Chief Executive Officer
It's really depend on customer. Some I have to say the channel probably react little bit faster and then there is enterprise then the service provider, sometimes take a little time because some service provider they also have to support in some of that, but we do see this [Indecipherable] a lot of new opportunity. And they also like these that tightly integrate approach whether SD-WAN or SASE which goes security driven networking which through are able then to offer additional service, addi