It's his age. My son acted the same way at age 4. I don't think you can 'blame' it on school or anything- he is just testing and learning to be independent, and unfortunately you are the person who is around to test!!
Age 4 is VERY young. 'Punishment' is a pretty harsh term for a 4 year old. I am not saying it is ok to allow him to talk back to you, but remember to be the adult- WHY is he doing it? I would be very careful about the potty training issue. If he is telling you to 'back off' when you are trying to make him potty, he must be feeling like you are really pressuring him. From personal experience, I know that is a BAD way to go- the more pressured he feels the more he will fight it.
We used to set a 'potty timer' and when it dinged, he had to sit on the pot for a few minutes, no matter what he was playing or doing. He didn't always like it, but he got used to it and it meant we didn't fight about it, which helped get him trained.
At that age, kids are little parrots, they hear something and repeat it back- their mouths and hands work before their brains can get into gear to think better of it. He is trying out this new phrase on you to see if he can get away with it- but that is perfectly NORMAL, so don't over-freak about it.
When he says " Back off!" or some other back talk thing, you need to be VERY CONSISTENT in how you and your husband deal with it. You are going to have to do the same thing, repeat the same thing over and over and over so it will sink in for him.
Giving too many time outs just makes them predictable and not that big a deal, so I would be careful how you use that 'punishment'.
1)Being consistent and dealing with it right away are both super important. Putting off something that will happen later is not going to work- it needs to be an immediate reaction.
2)Stop what you are doing and look him in the eye. Take his hands and say calmly and clearly " You just told me 'Back off' . We do not say that in our family; it is rude and mean. I don't want you to say that. Now we are not going to watch your show because you were rude to mommy"
3)Make sure he understands that HOW we say something is as important as WHAT we say. A little later, when you are having some quiet time together,repeat 'back off' to him in the EXACT same voice he used to you. Talk about a mean sounding voice or a nice sounding voice and that it is NEVER ok to use that mean voice with mommy or daddy or whoever.
Have this little talk over and over and over - not making it a big deal, but just repeating it whenever the situation comes up or every couple of days. It is just going to take repetition for it to sink in.
4)Get your son in the habit of acknowledging what he said and APOLOGIZING for it. Tell him, " When you say that in a mean voice, it hurt my feelings. I would like you to apologize to mommy and promise to try hard not to say that again" Then have him say he is sorry AND what he is sorry for ( saying back off, talking in his 'mean' voice, etc.)
None of this is going to make him perfect- all he can do is TRY, and remember, trying is HARD for a 4 year old! But this keeps it in his mind, makes him realize what he did- and hopefully trains him not to just blurt it out next time!
Honestly, I have to come out and just say DO NOT 'punish' by cutting back on reading books to him!! Reading aloud is THE #1 way to make your child into a proficient reader. Don't short change him on that just because of some idea of punishing him. Cut out TV or an ice cream treat or a toy- ANYTHING but reading time!!!
IMO, the same thing goes for playing with his daddy- most kids don't get to spend enough time with dad as it is, and dad has to be the 'heavy' when the kid is in trouble- why cut out any of the good time they have together? Get dad on the program with the little talk about what we say and how we say it.
If your son mouths off to him, have dad stop playing immediately and say "ok, now we have to stop playing and talk. Let's talk about what you just said to me" go through the drill. If he feels like your son was listening, etc. then have him say " If you understand and are going to promise to TRY not to say that again, we can play some more. Can you promise daddy to try hard to watch your mouth and not say that again?"