Engine balancing

The tricky bit is understanding that "unbalance" effects can occur at frequencies corresponding to 1/2 the crankshaft speed (sometimes called the 1/2 order balance), 1X the crankshaft speed (sometimes called the primary balance), 2X the crankshaft speed (sometimes called the secondary balance), etc. that can be strictly related to the selected configuration (i.e., the "unbalance" isn't caused because some of the components have different weights -- it's related to how the forces from each cylinder either cancel each other out or add together). Having the power pulses equally spaced ensures that the 1/2 order unbalance is minimized, and almost all IC engines have this characteristic (although, suprisingly, some don't). If you had one piston that was significantly different in weight than the others, this would cause a primary unbalance (i.e., vibration at a frequency corresponding to the rotation frequency of the crankshaft), but the geometry of the situation can result in secondary unbalance. I didn't search extensively, but this article:

http://www.absoluteastronomy.com/topics/Straight-4

does a pretty good job of describing why inline 4 cyl engines inherently suffer from a secondary imbalance. Try searching on terms like "engine secondary unbalance" or "engine vibration unbalance" or "engine balance shafts" (which is intentionally adding another mechanical imbalance to help cancel out the inherent vibration) on the web.