To quote from the ultimarc site
"The I-PAC2 has 28 inputs. They are all programmable and are designated as 2 joysticks, 8 buttons each, plus coin1, coin2, start1, start2. One of the inputs is assigned as a shift key to access an alternate code set."
"The I-PAC4 has 56 inputs. All programmable and designated 4 joysticks, 8 buttons each plus coin 1,2,3,4 and start 1,2,3,4. Two of the inputs are assigned as shift keys to access two separate codes sets covering the player 1,2 and the player 3,4 inputs."
So if you want separate imputs for all your joysticks and buttons get the I-PAC4. If you want to double up on your player one and have both the 4 way and 8 way on the first player then get the I-PAC2.
BobA