It sounds like you want to handle 3-4 RS-485
units through one "8051" serial port, isn't it.
RS-485 summarily:
- Balanced interface
- Multipoint operation
- Up to 32 Transciver Loads (Unit Loads)
- 10Mbps max. data rate (40 feet)
- 4000 Foot Max. Cable Length (100 kbps)
- ....
The hardware connection is not so difficult
(you can think about input protection in
noise environment and fail-safe requirements
and choose available driver and accurate HW connection).
More dificult is:
- the choice of appropriate communication protocol
(include unit addressing,...)
It can be random acces protocol (more difficult) or
Master-Slave protocol.
All your choices depend on application properties.
See:
http://www.lvr.com/serport.htm
http://www.rs-485.com/links.htm
http://www.rs485.com/plinks.html

HTH,
VK