well the 4.5 month old should be no problem at all! A kitten to most puppies is just great, I'm sure they'll get along just fine and will probably even play together. There might be some fur puffing at first, but once they sniff each other out it won't be a big deal I'm sure. Keep in mind those are really young kittens, and a little rough-housing will go a long way for them.
From what I've seen, boxers tend to be really good with cats. I would still be a little more careful with the 2yo if he's never been around cats. Make sure he's really calm when you introduce them. I would even suggest waiting until he finds a place to fall asleep, then just as he's falling asleep (i.e. VERY calm!) bring a kitten to him. Don't let him get excited about the kitten, he should no associate cats with excitement. Let him sniff the kitten, and as long as he's calm and almost uninterested in the kitten give him praise and let him know how awesome he is.
Even with the puppy, make sure he's calm when he meets the kittens. He'll probably be just as scared or concerned about what they are as they will be with him. Let the kittens meet him first so they get the smell and realize it's not a threat (from a smaller, less threatning animal) and make sure to take your time. Don't feel like this has to be a fast intorduction, just keep slowly introducing them to each other and use common sense, if it seems like your older dog wants to tear them to shreds then maybe you should wait until they're a little bigger to assimilate them, when the kittens can give him a big old scratch on the nose to let him realize that they won't take his pushiness! Also, dont' let the kittens hide under something where he can smell them. That makes it more of a game, "get the kitten from under the couch" and makes them seem more like a toy or something to hunt.
Dogs and cats can get along great in the right situation. When I first got my kitten my younger golden just LOVED him to pieces and they would rough-house like crazy (this cat did think it was a dog though haha). They'd bite eachother, Gregor (kitten) would latch onto her head and bite her ear, she'd pick him up and throw him...I know, it sounds gruesome but they loved it!! Unfortunately we couldn't keep the kitten (didn't understand the idea of litter boxes...seriously, he thought he was a dog) and had no interest in going outside (he was born in a barn and I guess that was enough outside for him) so it wasn't a good match.
Good luck! Let us know how it goes.
