Question: what is the difference between 2 pin and 4 pin lamps?
Answer: 4-pin varients are for just plain lamps - (just like linear flourescents, a filament (electrode) at each end, two pins for each) - 2 pin types have a (usually glow-type) starter switch built-in - connected in series between the two filaments - so you only get the two 'outer' connections.