The only way you can do it it to have different external ports mapped for each seperate PC..
For example
55550/1 goes to 55554/5 on 192.168.0.2
55552/3 goes to 55554/5 on 192.168.0.3
55554/5 goes to 55554/5 on 192.168.0.4
Though I'm not sure even some routers can map..instead the only forward