I have had problems using standard TTL for the 74C14 but don't really know why. What I use is
a CD40106. This really doesn't sound like the problem though because the other 3 reels are being
decoded and they all use the same chips.
When you move reader 4 to a different slot, does the error code change to 4x, where x is the new
reel location? If it does change then that reader board is bad, if it doesn't then I would suspect
a bad connection on the connector.
Somewhere you mentioned being able to see the lights on the reel readers being lit up so this would
indicate that you have the 50 volt reader cards. While the bulbs may light, the photo-transistor behind
one of the bulbs could be bad, however the bad reel reader problem would follow the slot.
I wrote a procedure to test reel reader cards an one of the board members allowed me to post
that on his website, the procedures are located at
http://www.coinslots.com/tips/bally-e-series/