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)
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.
Just like the MacBook Air line, the retina MacBook Pro line has no distinct RAM slots. This is clearly stated in the tech specs as well as the user manuals for your model.
Why not ask a follow-on question documenting the memory pressure you are seeing on the Mac and perhaps we can assist you in not needing to sell your Mac and buying another with a larger RAM capacity.
Your mac shouldn't be freezing. Do a apple hardware test or take it to an apple store.
Apple Hardware Test: http://support.apple.com/en-us/HT201257