This is on the level "Clean the Ancient Statue" and I had this unlock at 160 stars.
The four buckets in the level are located as follows and just use the red/narrow nozzle to knock them over.
1. As soon as you spawn in, past the RV over on the left2. Further along on the left from the first bucket3. Behind the statue, over on the right4. Behind the statue, right at the top of the wooden platformsLet me know if there is anything I can do to improve the solution!