Sorry, but why does the dad being home make it a more significant problem? She shouldn't be doing it in any case. (Are you saying you suspect a sexual connection? If she does it all the time, regardless of whether you are with her alone or her dad is home, I wouldn't suspect that.)
Maybe better to catch her before she does it and take her into the bathroom.At 4 year old you should be able to tell her No ,how do you mean'even when her Dady is home for work, does she do it when he isnt home ?