Telling my child her dad’s in prison…

I’m not sure if I should tell her the truth or keep up the “dads gone somewhere where they can help him get better” lie… I hate lying to her and I know she’s old enough to understand (she’s 4). She hasn’t seen him for 4 months and I know not being able to see him upsets her. But I don’t want to upset her more by telling her the truth. What do I do?

Also, if I do tell her will it benefit her to go see him at the prison? I’m not a big fan of taking her to an institution to see her dad. I don’t really think it’s a good image for a child to witness. But I have an open mind and if I can see some sort of benefit to it I will take her.