In regards to the fourth display not been detected, have you tried the other remaining ports? Have you swapped the adapters to verify if the adapter is the problem?
Always make sure as well that Windows remains up to date and also make sure that your mainboard BIOS is up to date.
Yes, you can use both cards without the DP port to power 6 screens. The R7 260X and R9 285 and 290 series can power 3 displays without the need of the DP port.