Put two more tube lights in fixture?
If there's a mounting location for a second ballast, yes.  Many 4-lamp fixtures do provide for 2 ballasts, and are built for same.  

Try to use modern electronic ballasts.  They are plentiful dirt cheap on eBay and Craigslist now that so many people are doing LED conversion. 

I would add to this, some electronic ballasts are actually made to drive 4 lamps.  The wiring can get a little bit knotty and you have to watch your wiring diagrams.