Your child has grown into a pre-schooler who will now socialize with kids of their age. This is the time when their interactive powers will be tested and refined. Hence, after some practice, they will be fit to pronounce some difficult words correctly.
What you need to know
As the child enters the age group of 3-4 years it is important for you to help the kid develop precise speech. Most children find it difficult to pronounce words starting with r, w, l, f, th, sh, ch and dz. To correct this you must train your child in a manner that keeps them focused. The first step is to ask the kid to pronounce such the word which is difficult for them to utter.
Let them try and make errors because that is when you act as a teacher. Make the little one understand the use of the tongue. Open your mouth and slowly show the kid where the tongue strikes when they pronounce th, r etc. Let the kid practice these alphabets or doublets first. The later lesson includes forming words with these sounds. You have to be very patient as it would take some time for the child to perfect their speech.