GREAT question! This is a question that parents ask me almost weekly. Speech sound acquisition is different in all children. With that being said, however, at least 50% of typically developing 4 year olds have all but two sounds that they can produce correctly. The sounds /r/ and “th” are typically the last sounds that children acquire; however, most children can produce the sound “th” by the age of 5 or 6. Assuming there are no other impairments that would be affecting your daughter’s articulation, she is at a great age to start teaching the “th” sound. In fact, this is my favorite phoneme to teach because it is a very visual sound. I suggest trying it at home first and if she continues to experience difficulty, then you should consider speech therapy. It is important to note that the “th” sound can be either voiceless (as in thumb, think, thank), which means it sounds like a whisper and we do not “turn on our vocal cords”, OR it can be voiced (as in this, there, that), which means it’s a louder sound and our vocal cords vibrate during production. Try these techniques to elicit proper production of the sound “th”:

1. Look in a mirror with your daughter and show her how to put her tongue between her teeth while blowing air at the same time. Tell her you want her tongue to pop out to say “Peek-a-boo!” Practice this sound as a whisper (without voice) and then louder (with voice).
2. Once she masters correct production of the sound in isolation, add a vowel to the end of the “th” sound. Try it with both voiced and voiceless “th” sounds.
3. Continue practice in the initial position of words and then gradually work your way to the final position (as in math, bath, teeth) and medial position of words (as in author, bathtub, birthday).
4. Consider giving her auditory feedback by recording her voice so that she can hear the difference between /f/ and “th”.   If you have an iPad, you can purchase single sounds from the application called Articulation Station by Little Bee Speech. This app facilitates practice at the single word level, sentence level or within stories. It features a recording option for auditory feedback and a data collection feature to help you monitor progress with individual sounds.