H.,
I had girls, so they were by far easier, BUT, a couple of strategies that helped us:
1) sticker chart with a reward at the end. Whenever he goes in the potty, he gets a sticker, and when he goes in his underwear, he loses one;
2) start throwing away his poopy underwear (be sure and get some cool ones that he wouldn't want to lose). I told my youngest, that "Mommy doesn't clean poo-poo panties," and after seeing a few of her favorite princess panties go in the trash (I even went so far as to making -her- throw them away), she got the message;
3) Several have suggested making him clean it up, and I always did that when my girls left a puddle, but as far as the poops, I'm thankful that I didn't have to deal with that a lot.
In my humble opinion, at 4 1/2, he's plenty old enough to comprehend actions and consequences. Make him deal with the consequences. If he leaks onto the floor, give him the supplies and make him clean it up, ignoring the fact that he has a load in his pants. If the mess is contained in his pants, just ignore it, so that he'll be forced to clean it up himself. If he comes to you and tells you that he's dirty, hand him a package of wipes and a trash can (for the wipes -and- the aforementioned cool underwear). Escort him to the bathroom, make him stand in the tub/shower, and let him find out just how gross it is to have to clean that up.
Best wishes and luck!
M.