Be aware of two things in this transaction:
(1) A lot of people find .40 to have the most noticeable recoil among the major calibers. Shooting a 9 mm and a .40 from the same subcompact is likely to feel a bit different to you.
(2) The subcompact member of the PX4 lineup does not have a rotating barrel as do the compact and full-sized. The rotating barrel softens the recoil quite noticeably.
Ultimately it is your choice. Just be sure you know what you are buying.