The standard answer to how much RAM i can install in my Acer Aspire One D255 is "2 gigs" and the standard reason given is "because it only has one RAM slot". Does this mean DDR3 RAM modules only go up to 2gigs? Do bigger ones exist or is there somethig in the DDR3 spec that limits the size? Wikipedia was no help this time.
Newegg seems to provide an ample number of examples of 4GB laptop DDR3 memory, which seem to be available across the board (at the two clock speeds).
To your specific questions:
No. It probably just means that your vendor (or whoever is answering your question) does not have access to 4GB sticks of RAM. I haven't dealt with aftermarket laptop memory, but installing two different capacity RAM sticks shouldn't be problematic.