check out some of maystar or magnats range of 4ch amps, would be perfect and they are budget amps but built very well for the prices. Or check ebay for a cheap second hand amp, rockfors or alpine or even phoenix gold small 4ch could be had for a decent price. Id say get some decent front comps then bridge 2 chanels of the amp for your sub and do without rears. Id recommend an RE8 or crystal comp8 if you can, both great little subs.