it all depends on your initial weight, when i did it i lost almost 6lbs but it all came back when i reintroduced solid food, not recommendable unless you have an specific event and need to lose weight as fast as possible
i think if u eat a little amount of food after your fast. like <500 cals for like few days after your fast i think perhaps you wont get a rapid weight gain, the weight gain would be waterweight anyways but still. just try your hardest to not binge after 4 day fast. im also gonna do a 4-5day long fast this week🤞