I think Plamb meant to say the power5 minimum is AIX 5.2 ML4.
So make sure your power4 system is 5.2 ML4 and you will also need to do a Migration install / restore to the power5 machine, so you will need IBM AIX install media (CD) of at least AIX 5.2 ML4 and it must be the same level / ML as the system yor are trying to restore.
When you need to load a power5 machine you need apars / ptfs to get it running as intended, so if you load 5.2ML4 or higher or 5.3, check the IBM website and pick up the latest patches once it is loaded.