Well, I'm assuming you know how to update your PSP's firmware system. (Why do I keep spelling it Fireware the first time I type it?)
If not, it basically involves you having an internet connection and going to the Network Update section of the PSP main menu, should do it automatically after that, that is, if it tells you an update is avaliable.
Either way, I'm assuming no more discussion needs to be done on the matter?
Just in case, I'll throw you this link.
How to update PSP Firmware.
That should about solve your problems if Firmware 4 does infact come with an updated Flash system, if it doesn't, then just assume that it's not going to work, and that it's not compatible or something.
Thread closed.