i 100% agree with craig,
get two, with one being a little larger.
4 will be too many in a 20, and they will get aggressive, i know they say percs are the most peaceful, but that doent mean always peaceful!!!
with all due respect, thats like saying, Nurse Sharks are the most peaceful out of great white, hammerhead, bull shark etc... but it will still bite your arm off if it comes down to it