I and V are designations for the configurations of the cylinders in the engines. An I4, also sometimes called a straight 4 has the cylinders lined up in a straight line. On a V4 you have two cylinders on each side of the engine lined up at an angle. The angle between the cylinders vary between designs and the angle is very important to the output of the engine, as is the V configuration. Another class of engine is similar to a V engine, but the cylinders are perfectly angled to each other, so that they line up flat. This is generally termed an opposed-4, flat 4, or a boxster engine as it was used in the Porsche Boxster. This flat type of engine is actually used in many 4cyl cars today.
A V engine has a cross-section (frontview) that looks like this: \/
An I engine one like this: |
An opposed engine has one like this: __ __