I would not be too concerned with this at her age, especially since she was speaking well up to this point. At her age, she is learning a lot of different language skills and her brain may be processing what she wants to say faster than she can say it. This is completely normal and she may outgrow it. When she is talking and begins to stutter, I wouldn't point it out to her, just be patient and let her get it out on her own.
If she does, however, continue to stutter for more than 6 months; if there is a history of stuttering in the family; or if she has other speech and/or language disorders, you may wish to have her evaluated by a speech-language pathologist.
This could very well be normal, especially if she was previously talking well for her age. Children at this age are learning and devoping their laguage skills.
As you know, the english language has many rules, some of which contradict basic phonetics. Your four year old's brain may be processing what she wants to say faster than she can say it.
Speech Therapists have an easy test that they can do to determine if this is the case. However, the main thing is not to draw attention to this problem. Do not tell her to slow down and speak slowly. Just listen and be patient. Chances are she will outgrow this soon.
Ask her if she has noticed it and if she knows why she is doing it. Ask very matter of factually. Her answer may surprise you. She may only be imitating her new best friend at school, or she could have been interrupted many times, or she may not be able to speak as fast as she is thinking, or something may have happened that distressed her. I'm not certain I would wait 6 months, though.