From a component standpoint, there shouldn't be any reason a 3 lamp fixture costs more than a 4 lamp fixture when it comes to T8.  Actually, the 4 lamp should be slightly higher given the extra sockets and labor for wiring.  A 3 lamp ballast is the same price as a 4 lamp ballast (at least from an OEM standpoint).  The only explanation I can think of is that with a 4 lamp fixture the ballast is located between the 2 lamp sets.  With a 3 lamp fixture you are forced to move the ballast somewhere else since there is now a lamp running down the center of the fixture. Maybe that means more material in the housing - maybe that means it's a completely different fixture which could be your supply/demand.

From an Electrical Contractor's perspective, I don't know where your friend is getting his information.  A 3-Lamp T8 Fluorescent Troffer runs about $68.00 with Lamps, a 4-Lamp T8 Fluorescent Troffer About $72.00.  The only cost difference is the extra lamp and sockets.  In most areas of the Country where we are concerned with Energy Compliance, 4-Lamp Fluorescent Fixtures are almost a thing of the past, as they are much more difficult to meet Lighting Compliance with.  Most major Lighting Manufacturers maintain huge stocks of 3-Lamp fixtures, and can ship almost any quantity in 24 hours.

Tom