sijp,
I did not understand that you wanted *both* computers to use amule. The setting i gave you is to be done in the p4, so that the computer *inside*, the p3 in your case, can have access to the ports. You can for example, change all the '4's to '7' and do the same change in amule prefs.
Like Jacobo said, you cant have both using the same ports, because one of them will forward ports for the other.
The first three lines are the port forwarding instructions, the second three lines are opening the firewall for the p3. if you want to use amule in p4 too, you will probably have to have other 3 lines for it.
But it seems to me that you were already using the p4 with amule. Do you get a high id with p4? If so, only the port forwarding lines will be necessary, but take care, because in this case you are on the internet without a firewall.
Any doubts, please ask, i know i have been a little confuse.

Cheers!