Not really the point if you specced and paid for 4GB though.

But then again, I'm not a hardware man.

A wise man once wrote thus:
Yes, it is annoying but then I suppose that they would argue that they've provided what I ordered and if I'd wanted 4GB for one computer I should have told them and they'd charge accordingly.

That said, if it's correct that the DIMMs marked K4D5108320-UCCC are SGRAM (i.e. the memory modules on them are designed for graphics use) then they could hardly have made them more incompatible if they'd tried.

you ordered online, you have the right to reject goods for any (or even no) reason

get them to make it right