inline 4. that means that the cylinders in the motor are arranged inline and there are 4 of them. inline is also the same as "straight" engines. other types are V engines. their cylinders are arranged in a "V." flat fours or horizantally apposed fours or "H" fours have the cylinders arranged horizantally instead of vertically.
when people say 3rd gen 4th gen etc. they mean the body styles that the type of car had at a certain time.