how to place decoupling capacitors on a four-layer board for through-hole components?
The best way to place decoupling capacitors is to visualize the current loop that your IC will create.

The purpose of the bypass cap is to shorten this loop. (Illustration borrowed from Macrofab).



The current doesn't really care whether it's traveling through a plane or a trace. All that matters is the loop length. Keep it short and you'll be fine. 

You can assume that current will travel in a straight line through your plane to reach the bypass cap. This isn't strictly true, but close enough for most purposes. 

For a more detailed explanation see this Macrofab post.