My stepdaughter recently turned 4 years old. She was raised by her mom and stepdad. Child was conceived during one night stand so paternity test was given but not until she was 3. Once paternity was established her mother did not allow her biological father to meet her. Now she has decided to let him be involved in her life. He has bought her gifts since paternity was established and sent her clothes so he tried to be involved until now. Her mother is not sure how to introduce him to her as her 'dad'. She knows her stepdad as dad but would like her to know her real dad. Anyone have any good advice on how to properly introduce her to her dad?

I would start VERY slowly. Start with home visits or have him over for dinner. Let her see that her parents don't have a problem with this person. THen you could try to get together at the park or playground.
I don't know that I would just say "hey honey, this is your real daddy". To her, he is a stranger. She needs to know that he is okay and it's okay to spend time with him. I wouldn't put a label on him yet. Let them build a relationship first.
When that is done - then you should all sit down together and explain it to her. You may want to visit a child psychologist and let them talk you through it.
Good Luck

I had almost the same situation... well actually I did. At 3 years old we finally got the paternity test and she was raised by and knew someone else as her Daddy. We introduced her biological father as "Daddy X". I have heard the term Poppy is cute as well for her bio dad. Just tell her she is a very lucky little girl who has 2 dads. But like it was said above introductions very slowly would be best. Her reaction to this will be largely based on how the parents handle this. Make it something to be wary of and she will be, make it something good and she will be good with it. My daughter handled it just fine. She knew she had two dads that loved her and we as her parents let her know that this was a good thing and we were supportive of both relationships. It is not a replacement person only and additional person.

Get together with mom and work out a "step up" plan. A schedule that gradually increases. Just as an example: Start with 2 hours with moms family and new dad all together twice a week (Tues & Thurs). Dinners (like mentioned), take her to the park together, go shopping at a mall, go see a kids movie. Then move it up to 6 hours alone with dad once a week (Saturday 10am to 4pm) and one 2 hour time all together mid week (Wednesday). Then increase that to 6 hours alone once a week (Saturday) and one 3 hour "dinner" mid week (Wednesday). Then increase to 1 over night (Friday 6pm to Saturday 12pm) every other weekend and 2 midweek 3 hour "dinners" each week. Then to 2 overnights (Friday 6pm to Sunday 4 pm) every other weekend and 1 "dinner" each week. Each new "step up" could occur about every 4 weeks. But each "step up" should only be implemented when the child shows they are completely comfortable with the one they are on and not showing any signs of transition trauma. But can be adjusted depending on how the child handles it to longer or shorter. It will seem hectic at first but will allow the child/dad to develop a relationship and then to have a consistent schedule of being together without any great deal of time away from wither parent.

Hope any of that was a help. Good luck!

Make a calendar and show her when she will be with each parent. That will help her to know exactly what is going on and what to expect.