I assume you had a one or two year lease agreement, and not a 4 year lease agreement. So your lease either went month to month at some point or you had to sign a new lease.

If you are month to month, then they can raise the rent anytime with 1mo notice. So regardless of whether it’s due on the 1st or 5th they can choose to raise your rent with one month notice. I’m not sure if them saying you have to “pay on the first or else” would count as notice or not, but it might. Even if it doesn’t though they can increase it the next month.

If you have a lease but don’t have or lost your copy you could ask the property management company for a copy. Not sure if they are obligated to give it to you, but unless they are really shady I would think they would.

At the end of the day if it’s not a big deal to pay earlier then you should probably just do it, rather than cause yourself more problems.