Not sure why some people assume that 4GB is enough, when we have lot of AAA developers asking for 8GB - and that's what they obviously consider to be minimum required if next-gen is to last as long as this one. Honestly, I don't think something like Halo4 would be possible if MS haven't increased memory to 512MB from originally planned 256MB, so even if they've planned for 4GB I'd bet they will listen to developers again on this one.
As for PC games not requiring 8GB - FarCry3 min requirements are 4GB, high-performance are 8GB - I expect to see lot of games with these sort of requirements in near future.