Nexus S 4G: Use wifi for LAN, 3G/4G for WAN?
by default, 3g/4g cannot coexist with wifi, 3g/4g will be disabled whenever wifi comes up, if you need this so badly you can use some custom rom's source and modify the below code to enable 3g during wifi, also you need to route like u already do.

http://gitorious.org/0xdroid/frameworks_base/blobs/234551c44102003c1fb2de5c16f60f9f0cefe71d/services/java/com/android/server/ConnectivityService.java#line224