How do you think the Mopho X4 would handle recreating some Radiohead/Thom Yorke type pad sounds?

It would do an extremely good job. Thom never uses all eight voices of the Prophet ‘08 at once, and the Mopho uses a very similar voice architecture (it has bonus sub-oscillators) and the same filters, so if you set the knobs right the two synths would probably be indistinguishable.