What I have read of Win 7, memory usage is different then it was in Vista. You can google Window 7 memory usage and find many articles on the subject. I recall reading at least one test where someone virtualized a system and kept decreasing the available RAM on boot. They managed to get Win 7 to boot with only 300 MB of RAM, but it was very slow. They concluded, correct or not, that the static memory limit was 300 MB. (Really a rough test.)
Many more discussions indicate under a typical running load the usage varies from 1GB to 1.5GBs--"typical" being Windows, maybe a IE session, and a couple other backround apps. Throw in a game on top of that and I'd suspect that with 4GB you'd still be able to run most games. Any slow down would likely be to demands on the GPU or heat, since heat is usually the biggest problem with gaming laptops.