Nice job , the other option i know its a bit late but it may help someone else.
You could use an electric liquid level controller, you can see in first picture a bit of conduit in the bucket with 3 bolts along it ,the bottom one in bucket is ground reference second start top is stop ,this is slightly different as im using this to catch water from my dehumidifier and a pump inside the bucket will pump it away so my on offs are the opposite on is top off is the middle bolt . So you would have them closer together and probably no need for 5th bucket you could link all 4 buckets and fit a probe in the bottom of one of them .
The probe is safe to touch .
Controller was about $12