There are a couple of possible problems here but I would need more information to give you an exact answer. You said it was a 120v 4-lamp ballast but you also said there were two ballasts. You also referred to one of them as the older 'type', by that do you mean one was electronic and the other magnetic? What type lamps were you using, T8 or T12? If you had a magnetic ballast and T12 lamps you may need to change both lamps that are fed by the ballast to get it to work, at least thats what I always do. If you have an electronic ballast and T8 lamps you can just change the bad lamp. BTW you should never mix electronic ballasts and T12 lamps or magnetic ballasts and T8 lamps. Like HID lamps, fluorescent lamps are ballast specific.
As for troubleshooting a fluorescent fixture, the quickest, easiest way is to change the lamps and if that doesn't work change the ballast. That will get your light working and you out the door in about 30 minutes.
