Without seeing this I'll just toss this out there...
Would seem a better train of thought to keep the bolts from coming out rather than detecting one of four missing? If the conveyor is designed for "x" load, I'm guessing one bolt missing on one side might be risking failure instantly before the eyeball would have a chance to have a look at an empty hole?
Maybe switch to an alloy pin with a head and put it in the hole... then drill through your slat and pin... then counter bore through the top side of the slat and through the pin stopping short of going through the bottom of the slat. Then smack a rolled pin into the counterbore to keep it in place... then ding the edge of the rolled pins hole so it can't back out. The through hole is so you can put a smaller punch in from the bottom to drive the pin back out if you need to change the slat.
Or if it's a bolt and nut through your slat maybe a locknut (I really like the steel insert locknuts) or a jam nut... or a different fastener altogether. Like the pin with a washer and e-clip...?
Far as seeing if the bolt is missing I'm sure somebody on here is well versed in those sorts of sensors and will chime in.
Good luck and pass out some hard hats... !!