To answer this question I guess we need to know the circuit construction details of commercial electronic fluorescent ballasts. Is there a downside (either power efficiency or ballast longevity) when connecting only 2 lamps to a 4-lamp electronic ballast?
Bonus question: The ballast is spec'd for T-8 lamps, but I intend to use T-12 lamps, so what is the possible downside of using T-12 lamps in a T-8 spec'd ballast? (I've been running a 4-lamp fixture in this mode for more than a year with no negative effects seen).
Maybe a more general question is - what exactly changes in terms of electrical excitation / voltage / current draw when a fluorescent tube lamp changes in diameter from 1 inch to 1.5 inches?