I have finally implemented the working network play feature in VGBA-Android 4.0, my Nintendo GameBoy Advance emulator released to Google Play a moment ago:

https://play.google.com/store/apps/details?id=com.fms.vgba

The NetPlay feature is still experimental, but proven to work in F-Zero and Pokemon games, at least. To use network play, select "Connect NetPlay" from the menu. Select "Disconnect NetPlay" when you are done. Both players have to be on the same WiFi network, or use global IP addresses. Remember to always save your game state before using NetPlay!