I have actually set up a brilliant setup with a similar budget just last year!
Your first priority should be the head unit, don't skimp here, because it will make a HUGE difference!
I would recommend the JBL GT-X333 (GT-X555 if you want to play CDs) Head Unit, it has 50x4 W Peak output.
When it comes to speakers, there is actually a noticeable difference in quality between 4" and 6", so I would recommend getting 4 JVC or Sony speakers if you can get 6" fitted. If your provision is for 4" may be able to fit 5.25" speakers, in which case look for Alpine or Sony speakers.
Also, don't get carried away by tweeters! Although they may seem tempting, you will be better off with buying 3- or 4-way speakers, which have inbuilt tweeters.