Well there is a way to split the device to make the device PBX think that there are multiple PSTN devices for each port. Im not sure if this will work with the 4104, but will be interesting to see if you do manage to tweak this little guide into making it work for your 4104. Same principals apply. This guide is for a patton model, and is not exactly a recommended or supported scenario by 3cx but none the less try it out and see how you go.
1. Upload the Config to the Device.
2. DELETE the Device from the 3CX Console > PSTN Devices
3. Go to 3CX Console > Settings > Provisioning templates > Patton model in question (Firmware R5.x) > Press Edit.
4. Search for "NumberOfLines" and replace the value of "4" to "1"
5. Press OK then press OK then press YES.
6. Click "Add PSTN Gateway"
7. You will now need to follow http://www.3cx.com/voip-gateways/patton-smartnode.html
and repeat this twice, once for each port. When presented with the port number "5060", leave this as is for the first port and set to "5062" for the second port, "5064" for the third port and "5066" for the forth port.
8. You may wish to revert the change made in steps 3 and 4 above to avoid complications in the future.
9. You can now setup your outbound rules to address each port individually.