Best way to explain

So for the last almost 3 years my fiancé has been unaware he has a daughter. He was told she wasn’t his and he also got caught up in some stuff that wasn’t good. He’s 100% clean and sober now, and now that we’ve done a official paternity test through the courts we want to be apart of her life. She’s almost 4 years old and we plan on going up next week to see her. What’s the best way to try and help her understand that he’s her dad? Also before you go judging my fiancé please understand she has lied to him for almost 3 years he tried staying with her but she ended up moving to another state 6 months after the baby was born and he did try and keep in contact with her to see his daughter but she kept her from him. They did a “at home” dna test when they first split and that’s when she said she wasn’t his. She then hit him for child support at the beginning of this year and never once attempted to contact him and ask if he wanted to see her. It took me finding her number online for him to even get a hold of her.