I love surefire.Carried sure fire in Iraq... My wallet hates sure fire. Get a streamlight trl 1. Super bright, LED, attaches to any picatiny rail system,and you can pick one up for under $100. It also has a set screw that let's you lock it down better than the M3 light.
If your m4 doesn't have a rail you can pick up a composite one pretty cheap. Measure the length of your handguards. I have a rail set that has a rail on all four sides I'd be willing to trade you.
Try La police gear, brigade quartermaster, or grey group. I can't remember which one but one of them just had them on sale for something like 89 or 90 bucks.