Are there any restrictions on using all 4 ports of the CP2108 in RS485 mode?
There is a bug in revision B02 of the CP2108 that occurs when using the DE signal of RS485 from ports 0 and 1 at the same time, or ports 2 and 3 at the same time. The bug causes the DE signal to sometimes be deasserted during transmission. The problem can be avoided by not using channel 0 and 1 in RS485 mode, and not using channel 2 and 3 in RS485 mode.
This issue will be fixed in revision B03.