Hi filosophizer,
I don't think a switch is the answer...
Your config has fallen out of the IBM "supported" / known to work, configuration area.
The DS is simply too old for your OS level - hence the weird results.
IBM System Storage Interoperation Center (SSIC)
AIX 6.1 TL3 was the latest level to officially support the DS4500 and then the DS had to have the latest firmware.
From AIX 6.1 TL4 the DS native (MPIO / RDAC) support changed to only support the newer generation of DS storage.
You might try switching from MPIO/APPCM back to RDAC/fcparray with manage_disk_drivers to see if that will still work - rmdev -Rdl the adapters and make sure the dacs/dars and disks have all gone before bringing them back with a reboot or cfgmgr to bring in the new (old) driver.
Make sure the DS4k5 firmware is the latest, 06.60.xx, before you go any further...
HTH