Your chest muscles are like stretched out rubberbands (horizontally). While you can target upper and lower, you can't really target inner or outer, it's all the same continuous muscle.
[quote]Here's a biology lesson: every skeletal muscle is made up of cells called muscle fibers. Each muscle fiber runs the entire length of the muscle. It is patently impossible to contract only one half of a muscle fiber. So upper vs. lower pec(toralis major) is fine, since the insertion of the pectoralis major is broad enough that the upper and lower pec muscle fibers (dividing the muscle along muscle fibers) produce slightly different mechanics. Inner vs. outer pec is not possible. The only other muscle that has such a wide insertion to really make a division within the muscle along (not across) the length of muscle fibers would be the deltoid (anterior, middle, posterior).[/quote]