Taken from sk44643
1. Copy the IPSO 4.x bootmgr to the system using winscp, ftp etc. The file should be in the format nkipflash-4.x.bin
2. Use upgrade_bootmgr to write the 4.2 bootmgr to the correct location on flash.
fw[admin]# upgrade_bootmgr nkipflash-4.x.bin (do not specify the boot device)
You may get an error here as the device is required; if so, specify /dev/wd0:
fw[admin]# upgrade_bootmgr nkipflash-4.x.bin /dev/wd0
3. Reboot and stop at the BOOTMGR> prompt.
BOOTMGR> sh
# disklabel -r /dev/wd0s4 > /tmp/label
# disklabel -R /dev/wd0s4 /tmp/label
# exit
BOOTMGR> install
4. Go ahead and do your IPSO install; when complete, reboot.