2 daddys? Opinion needed!

Hello, I have a 4 year old daughter with an ex boyfriend of many years ago and I'm currently ttc #2 with my husband of 2 years. My ex and I live in separate areas of the country and my daughter knows my husband as her daddy due to her biological father coming in and out of her life. Once I got married, he took off again and I didn't hear from him for almost a year. So my daughter ends up calling my husband daddy and it's the best choice. So now, he has spent almost a year trying to keep in contact with me and trying to change and be apart of my daughters life again. I'm hesitant to let him back into my daughter's life for more than one reason but if I do let him back into her life...how do I get a 4 year old to understand she has 2 daddys? Or how do I find a way for her to call her biological father something other than daddy? Any ideas are appreciated!!!!