Is this maintenance that is actually occurring, or the PM is taking out an additional $400 a month into some sort of reserve or as some sort of contingency?
If the former, it's difficult to say whether it's normal or not without knowing a lot more about the size, condition, and type of property we're talking about. If the latter, then I would say it's inappropriate as it's not the property manager's job to ensure you have adequate reserves, it's yours. Since it's 4K in rents and the amount quoted is $400, I'm guessing the PM is trying to enforce your keeping a 10% of rents repair budget. While that number is probably appropriate in a lot of cases, and may even be in yours, it's not the PM's job to be withholding those funds. All of this is under the assumption that this money is sitting in a reserve account, and not being permanently paid to the PM.
You signed a PM contract, what did it say?