Hello,
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I will try to answer all of your questions. Stuttering in 90 percent of the case is a learned behavior or a habit developed due to various circumstances.
So, in the first part, you mentioned that he is almost four years. Now, this is developmental stuttering according to research, most of the children around a similar age start stuttering out of those kids who stutter 80 % recover on their own. This stage is very important as a lot of concern on the carer part can put him in the habit of stuttering regularly.
In this stage, I would advise you that you reduce your speech (your speed of talking). Try talking slowly and encourage the child to speak slowly too. Kids tend to copy their parents and their style of speaking. Now, if the carer has a fast rate of speech, the child might try to copy, but the problem here is as an adult you have good vocabulary and good lung capacity so you can quickly say what you mean, but kids at age 4 do not have such a strong vocabulary thus either they will repeat the words or stop (people interpret it as stuttering). Many a time when kids get too excited, they tend to forget what they are supposed to say.
Now coming to the second part, where you said about different languages he has been exposed too. Generally, at such a young age, we encourage to have a basic language. However, he is exposed to two, which is pretty common in India. But I will advise you to strengthen the two languages he has before he starts. All the different languages have different speeds of talking, so it can be difficult for him to grasp it at such a young age. One of the factors which can cause him to stutter is the language mix up. When the child is trying to express, he might be shifting in two languages, and the vocabulary might be difficult for him to retrieve. At an early stage, please keep it at a minimum amount of languages. As he will grow old, he will pick up Tamil. But let him take his time learning languages.
Now regarding therapy, try getting in the habit of slowing down your speech rate and encouraging the child to do so. See if there is a difference many times, kids do stop stuttering. But if there is no change after two to three months, consult a speech therapist nearby or online, whichever you prefer. If it is stuttering therapy, it is not essential for a speech therapist to speak the language if the primary person who will train your child will be you. Suppose the speech therapist insists on taking therapy by themselves its essential for the therapist to know the language which is comfortable for your child.
In my opinion, train the carer because the kids are more comfortable with their parents than a stranger and thus, I have given therapy to a Chinese child as I trained their parents to help him out. It all depends on the understanding between you and the speech therapist.
I hope this helps.