i have 4" inch front speakers and a big 12" in the back, it sounds good, but you miss out on mid bass. But i buaght the best 4" i could find which ended up bieng fusion 120watts with adjustable tweeters.
Your best options would be to get an 8" subwoofer which covers mid bass and is a better match for you 4" speakers, or put 6" in the back to cover mid bass and let the subwoofer cover the deep bass. Problem with that tho is that your soundstage becomes less then ideal. Deep bass doesn't matter becuase you can't tell where it's coming from, but midbass you can tell where it's coming from and it will come from behind. That is how my setup is at the moment but it's sounds good still. if your running your 35watt 4" off your headunit there's no point getting higher power 4" speakers.