Both platforms can run in IPS mode with 5.x software, but you must have the optional 4FE card (IDS-4FE-INT=) installed to do this. You can install and run 5.x software without the card, but you will only be able to run in IDS mode since you will not have enough sensing interfaces to create an inline pair.
Let me know if this does not answer your question.
-Rusty