Why NEO4?? I have been using a neokey devices since getting my PS2. Before I go further - I think the work that the NEO crew do is fantastic and welcome any new development. A single solution to all our problems is what we all want.
Neo 4 has a lot of connections that if not installed correctly are potential problems. A sha** handed installer has more chances of making a mistake - so if you go this route make sure you get a recognised installer to do it for you or someone like myself who designs board for a living.
The NEOKEY has one link wire in. The only swap is for PS2 backups using the Action Replay 2 and this is not limited by time. EA sports games do also work providing they have been patched. A down side to this method against NEO4 was DVD RC. This is now rectified using the latest Action Replay 2 as it contains DVD'X' inside it and retails for £30.
The neokey will cost you around £20. I can confirm that it works on all models so far BUT I have not personally tested it on the SCPH 3xxxxR models.