When you start capping leads you increase the ballast factor of the system. This increase the current and the wattage drawn. From an energy standpoint, it's a bad idea. It's going to cost you money. From a lamp warranty standpoint, you don't want want to overdrive them too hard. Most manufacturers don't recommend going over that 1.18 BF threshold and you certainly will driving one lamp with a four lamp ballast. You'll also create more heat doing this.
There's a reason why companies make ballasts for differing amounts of lamps.
Some revisions of GE UltraMax uses individual lamp current regulation so the BF remains virtually constant when de-lamped or lamps fail, which they call Active Current Regulation.
(not all same model number ballasts are the same. Different product number with same model # can have different design with GE ballasts)
A typical four lamp 0.87BF ballast will run at 0.95 to 1.00 when running 3 lamps and 1.1 or so running 2. Many 4 lamp T8 ballasts do not allow more than 3 40W 5' T8s. Running four F40T8 will cause over-wattage and it can burn out.
The ballast label only shows a few typical examples of configuration. Some ballasts allow N-2, some don't. You often have to comb through manufacturers ballast catalog or request detailed datasheet from technical support to see all configuration. That said, some manufacturer sanctioned configurations do list BF as high as 1.3.
If you're going to do this, its important to print out the brochure and keep it on file. I recommend sticking with the same brand as well.
If you buy OSI ballasts and run OSI lamps at documented factory sanctioned configuration that deviates from warranty criteria, such as running at 1.3 BF, they have nobody to point fingers to and they're going to have hard time denying warranty as it proves you followed their own recommended practice.
Say you run OSI lamps with a 4-lamp GE ballast that permits running two lamps but runs at BF 1.3. When a bunch fails and OSI asks for ballast for warranty inspection (inspection of ballast for warranty claim investigation is often permitted by warranty contract) they'll likely say its not their problem that their lamps failed due to GE ballast running them outside of ANSI recommendations.
and i'm told to use a 4 tube ballast for all app's, yet when showing the bean counters the different bulb configuations right on the ballast, it clearly shows a 4 tube ballast feeding a 3 or 4 tube Florissant and a 2 tube ballast shows it feeding a 2 tube or 1 tube florissant. does any one of my fellow brothers/sisters have any info or links that can hold water to my dilemma 
