First I want to say welcome to YT!!! and Congradulations on little Raja
Yorkies are so different than any other breed that I have had too. Raja is still a puppy and it is not uncommon at all for them to be a little leary of new people. Imagine if you were his size
It sounds like you are doing a good job with him. Do you take him out much to meet new people? I think that four months is a good time to start. and you can take him out more in larger intervals as he gets used to this. Once he has all his puppy shots the park is a great place to see people but not be overwhelmed.
Also I would give the new people that come into your home a treat to give him so that he associates people as a good thing. If people = reward he will learn to look forward to meeting new people. You could try this in your play group too. Give him time and I am sure he will grow out of alot of it