Can I use a ballast that is rated for four 32W T8 bulbs with a different number of 15W T8 bulbs?
No. A ballast provides a method of creating an arc to ignite a plasma; once the strike happens, the resistance of the tube dives, so the ballast controls the maximum wattage. If one lamp failed to ignite, the other lamp would be allowed to be driven at 2x the current, and this can cause the driven lamp to explode.
Each lamp needs to be controlled by the proper sized ballast.