I'd say you're got a while to wait anyway kmg. Your 4 year old is probably a little too young to get the hang of a PS or GB. If the cost shocks you, try looking for secondhand stuff. You can pick up a used gameboy colour for about £30.00.
I don't think I've yet met a parent who wholeheartedly approves of gameboys and playstations. Never known one who urges their child to give them a go at any rate
My son got an X-box because his friends did, and he got used to playing with them on playdates. At age 9, if he didn't have a playstation or X-box, he'd be the only 9 year old boy I know of around here without access to one. I think that would get to him and I don't disapprove of them enough to say no - you will be different from all your friends. I have to admit it is fun, it holds his interest and it's great for rainy afternoons.
Happycat, I'd vote for gameboys. I think they are a bit easier to get the hang of and it's a real blessing that they don't need setting up. Your sons can use them on journeys whe they wouldn't be doing anything else with their time anyway. With a Playstation, any time they spend on it is time they won't spend playing or going out. However, unless your sons are really nagging for gameboys/plystations you could wait a while longer or just buy one for your 7 year old and see how it goes.
My son had a gameboy when he was just over 5 years old. He really didn't start geting into it till he was nearly 6. Also, there are onscreen instructions to follow - and they make the games more interesting - so it depends on reading ability as well.