Having looked on CW I am not sure they have been spawned in a tank? the link on there goes to C Araguaiaensis, so I assume they are similar in breeding?
Try a 50% cold-water change, reducing the temperature by 10°F (5.6°C) down to 66°F (18.8°C) to see if that triggers them.
Can you take a pic from above? its fairly easy usually to sex them when adults as the females are a lot fatter.
Dont assume cos you have four you have both sexes, I knew a guy with 12 zebra plecs and every one was female:wb: