Ok, for AIX 4.2 it will be trial and error to find what commands and command options are available because it is so old.
I found the AIX 4.3 command reference here so we'll hope not too much has changed from 4.2 and use these docs for now:
AIX Version 4.3 Base Documentation
(They actually seem to have a better interface and a much faster site than IBM can manage for the recent documentation!)
So your fsck options are here:
fsck Command
Before you go down that route you should try the df command to check the boot failure is not simply that a filesystem like / or /var is full.
Also look at the errpt output:
errpt -i <path to original error log: /var/adm/ras/errlog> on the mounted harddisk rootvg.
For the df and errpt checking you will need to boot the install cd and start the maintenance shell, then mount the hard disk rootvg.
HTH