Yeah you can put more than 4gigs but as Tedster mentioned a 32bit Version will detect approximately 3.5Gb maximum so if you have 4Gb RAM you will need to buy a 64bit Windows version any XP/Vista for it to detect and use all the RAM.
So if you don't want to buy another OS (64bit) then I will recommend you a 3 to 3.5GBRAM and don't mind Vista will work well on 3GB(if you will use Vista) and Win XP will be fantastic.