Can I connect 4 wire and 2 wire RS485 devices on the same network?
4 wire device provide full duplex communication and 2 wire half duplex communication. It depends what kind of transceivers you are using and what communication you would like to have (half or full duplex).

It is possible to use 4 wire transceiver on a two wire bus. 
With more information in your question you would get much better answer.