From website: http://www-1.ibm.com/support/docview.wss?uid=isg1SSRVAIX53TIPS081511_181
Due to an error in POWER4 system firmware exposed by an increase in the boot image size of the AIX 5.3 5300-04 Technology Level, some POWER4 systems may fail to boot after installing the 5300-04 Technology Level. To workaround this issue, perform the following steps.
setenv real-base c00000 dev / reset-all