Yeh that sounds arite, although I'm not too sure about 4" speakers in the front.
You want your best speakers in the front, (and you want to spend a third to half your budget on them), for this is where you sit and you want music to come towards you, not from over your shoulder. When your at a concert you don't expect the music to be coming from behind you, you want it in front of you!
Ideally you'd want to put 6.5" components in the front (2 6.5" speakers, and 2 tweeters), for they offer 4 differently postioned speakers giving out different freq ranges, making them sound superb, the sound clarity you can get out of top range sets is amazing. But I understand your problem with mounting them.
I've never used 4" speakers so I can't comment on them too much, I can imagine they're freq range would not go as low as 6.5s however, and may suffer on lower midtones. Your 6.5"s and sub in the back may well compensate for this however. And for a low budget system, they will probably be fine.
Definitely better than most people's brainwashed "two subs and 4 6X9s, all mounted in the boot" approach
