It sounds like you must be a very loving and patient mommy :O) Well, just FYI, I have 2 boys, and they can say sorry very nicely, so it can be taught. We just need to help you figure out how :O)
Do you raise your voice to your boys? It almost sounds like you are very "nice" and do not raise your voice to sound serious enough. I don't think your boys are taking you seriously. I think it's time to show them "who's the Boss". You don't need to YELL at them, just change your "mom tone" to be serious.....we all have it in us :o)
It sounds like you have 2 issue's. The first one is behavioral (grabbing, etc...), and the 2nd one is remorse, or lack of. They seem like they go together, but your son needs them seperated for awhile. This way you can focus on the behavorial problem, then hopefully the "sorry" feelings will come naturally.
1st off, find a new time-out step/chair within your eye's reach. Then focus on giving your son 3 reasonable warnings, like "Son, you will not grab toys from other people's hands,you need to use your words and ask nicely to play with it....next time you are not nice to your friends, you will sit in time-out". Then, "Son, this is your second warning...." while you're showing 2 fingers and repeating what he SHOULD'VE done. Always remind him that he is headed for a time-out. This gives him a chance to hopefully make a better "choice".
If the time-outs do not work, then you will need to take something else away, like when everybody else goes out to play, he has to sit and watch in a chair, or whatever you can think of that will hopefully reduce the need to misbehave.
This consistency will take a week or so to see any difference. But your son is almost 4, and needs to learn how to keep his frustrations to himself and not to others. Otherwise, you are headed for the Principal's office in a couple of years.
Simply teach (or re-teach) him how to use his words to express himself. If you are close enough to him, then you can always hear him, and help him BEFORE it gets physical. This will take alot of focus on your son, which will probably be difficult, but it is very necessary.
When he does the "right" things, don't forget to hug him with praise. Hopefully he'll want more of those and choose to be good all of the time :O)
Good Luck, H.. I hope some of this helps you :O)