Usually you only see roller bearings in the bottom ends of 2 cycle engines, how else would the fuel-oil mix suspended in air lube the bearings? (*think* roller bearings, or ball bearings, will not hold oil under pressure worth crap, to lube the rest of the engine)
The "Usual" for 4 stroke engines is a a "plain" bearing, or a "babbitt" bearing:
http://en.wikipedia.org/wiki/Babbitt_%28metal%29
The main feature is this bearing will hold oil pressure and allows the entire engine to be lubricated with a single, simple, oil pump.
Some hybrids do exist, but they are few, and far between, and certainly not in most of our price range.