I have successfully completed the 3 day juice fast. Can I take alcohol on the 4th day?

No. Alcohol is not a part of the Satvic lifestyle. The juice fast has done so much cleansing, even a little bit of alcohol will take away all the benefits of the fast, and make things worse.

We suggest you use this three day juice fast as an opportunity to live an alcohol free life.

Please follow the rules of breaking the fast very closely as given in this other answer.