Can I upgrade RAM on an early 2014 Macbook Pro with Retina display? I bought a Macbook Pro with Retina display (4GB RAM, 128GB SSD) in April.

It keeps freezing and about every 3 days, and I need to upgrade the RAM.

Am I able to upgrade the RAM on this machine to 8GB or 16GB?
No, you cannot add more RAM.

Open up Activity monitor, located in the Utilities folder of the Applications folder (or just search using Spotlight)

/Applications/Utilities/Activity Monitor

Go to the memory tab and check out the memory pressure and swap used. If the memory pressure is too high (yellow/red graph) you can figure out which applications are using the most memory by going to the View menu and choosing Columns -> Real Memory. Sort applications by the newly added Real Memory column and you will see which applications/processes are taking the most memory and slowing/freezing your computer.