My 4yo does this too!!!! Not all the time but sometimes and it drives me batty! She tends to do it if she's feeling shy or awkward about something but sometimes, she does it for no reason.
If she's feeling shy or awkward, I don't make a huge issue out of it but I do tell her I can only understand big girl words.
If she's doing it just to do it, I ignore it and say firmly "Big girl words only, please" and I pretened not to understand her until she speaks in her normal voice.