I find it interesting that you bought an IPAC/4 for a 2 player CP. The IPAC4 is designed for 4 players (hence the 4). The IPAC/2 is generally used for 2 player and most people would setup the IPAC2 with 2 8 ways and a dedicated 4 way as you say (same connections as player 1 8 way). I'm not sure if there would be any good reason to not set it up this way now even with the IPAC4.
So what I am trying to say...yes....hook up the dedicated 4 way along with one of the 8 ways on the p1 side of the IPAC4.
If you aren't planning on creating a 4P CP you might should look at trading your IPAC/4 for a IPAC/2 + something else...