What speed is my Shimano Nexus 8 Hub in? I made a mistake when changing a flat. I didn't have it in 4th speed. When I was changing the flat I shifted it for more give on the shifter cable >_&lt; . Now my shifter says 4 and I don't known what the gear is actually in. 

I've been field testing it to get the 4 on the shifter to put the hub in 4th speed. I'll commute to work, experience slippage, adjust it so the yellow lines match, and continue experiencing slippage.

I'll unhook the shifter cable. Shift down so 4 is now 3. Field test again.

How do I know what speed my hub is in when the shifter doesn't match?
Put your shifter in 4th gear. Adjust the cable tension so that the yellow lines match. There really isn't more to it. Not much to do wrong. If you can then shift all the way up to 8 and down to 1, the basic installation is correct.

However ... from my experience, these hubs change a little over time. Mine (Alfine 8, basically the same thing) shows slippage in some gear every time I re-install it. After aligning the yellow lines, try to slacken the cable a little bit. This could be closer to the setting the hub was before you changed your tire.