grssslotrepair,
First of all, what model machine is this? Are you able to see the 5th. click arm when you play/cycle the machine? If so, when you play the machine, you should see the payout slides being pushed forward and the 5th. click arm will move over and hold the payout slides forward briefly. At the end of the cycle the 5th. click arm will move out of the way allowing the slides to "reset". That sound of the slides resetting is the 5th. click sound.
You are correct that the payout fingers hitting the payout discs is the 4th. click. Slide reset is the 5th. click. I believe that your 5th. click arm may be gummed up/sticking causing the payout slides to reset too early.
Also, as shortrackskater stated. the coincollectorforum.com is a good source for the older machines. Just google "5th. Click" and you will see a few postings regarding similar issues as yours.