If you are only taking one shot at a time the CH 4FPS setting should have no effect. The specifications for your flash say it can take up to 3 seconds to recycle. The inconsistant exposures you are getting sounds like you are taking a shot before the flash fully recycles.
You might consider increasing the camera's ISO to see if you can reduce the flash output and get quicker recycle times. Try ISO 400 and see if that changes anything.