Can't upgrade my Mac

I bought a new Mac with retina display (4gb/cheapest retina version). I used it for a month without any problems, but after that it started freezing/crashing. I tried contacting apple saying this is not what I expected (with 4gb of ram) with all the crashing's and they said they couldn't do anything about it since it's past the return date. I can't even upgrade the ram (which is the reason for the freezes). Now, they come out with 8gb ram and faster ones for the same price. I can't even sell this for as much as I would have liked either.

Shouldn't they be able to help me get 8gb of ram, since my Mac doesn't even function properly?