Well, at 4 months he's still just a baby, and they're experts at pooping and sleeping. I always wore an older shirt for the first few months when I got my little guy - that way if he answer nature's call while snuggling, I could just toss the shirt in the sink, let it soak, and grab a different old shirt.
I'd also recommend wearing an old shirt for a day or two and putting it in the cage with him. This will let him get used to your scent even when you're not around and will help him associate the smell with comfort and safety. I used to wrap Norman up in my shirt before putting him back in his pen to sleep.
The best way to get him used to being around you is to just spend some snuggle time with him. Take him out of his cage, sit down on the couch, plop him on your lap, and give him an old shirt to hide in. Give him some time to just settle in on you and get comfortable. He still needs to sleep a lot.
When you pet him, keep petting him even when he raises his quills and starts huffing. If you stop, he'll eventually figure out that doing that will get him what he wants.
As for how big he'll get, there's no real standard. Hedgehog sizes vary just like people. My little guy is 11 months old and only 360 grams. You can take a look at the Average Hedgehog Size and Weight
topic on here to get at least some idea as to how big he might get, but it's not something you'll really know until he's actually full grown. As long as he's not so plump that he can't form a tight ball or really skinny, he should be okay.