[Theohazard]

To answer the OP's question, there are three major influences on Form 4 (and Form 1) wait times:

1: Backlog.
Depending on how many total forms they have in their system compared to how many examiners they have on staff, the wait time will go up or down. My last two Form 4s were submitted within 2 months of each other, the first came back in 4 months and the second came back in over 7 months. And that went along with the trend I saw at the shop I where worked at the time. So in just 2 months the backlog went up by about 3 months.

2: Trust vs. individual.
I hear ATF is currently processing forms submitted as individuals faster than forms submitted as trusts.

3: Random chance.
Sometimes paperwork gets shuffled around and some examiners are better than others. At the high-volume NFA dealer where I used to work, I saw more than one Form 4 that was signed and approved way before we got it back. That means it was approved and then sat on someone's desk for sometimes up to two months for no apparent reason. This was relatively rare though, much more common was just a seemingly random approval time fluctuation among forms sent in at the exact same time. Two customers could each buy a silencer at the exact same time, and it wouldn't be unusual for one form to come back a month or two before the other.

There's no reason to worry and no reason to call them and waste their time. If they've cashed your check then your form is in the system. Now you just have to be patient and wait.