Roland UA 4FX is simple stereo USB audio, so all you will see is 2 inputs or L & R.
Plus, Prime edition only supports 2 inputs/outputs.
You can use a DI box to slit the signal.
1) Guitar to DI box
2) DI XLR Output to XLR Mic input
3) DI 1/4" Thru to iRig2 > iPad > iRig2 > H/P Out to Left or Right RCA only.
4) Then add 2 mono tracks in Prime, set one to input 1 (L) and the other input 2 (R).
I' can't confirm if both Mic input and Guitar 1/4 input go to both stereo or onto one channel.
Hope this helps.