S.H.
Don't worry.
He is normal.
MANY kids get like this.
Do NOT compare him to other kids in the vicinity.
My kids, BOTH of them, were like that when younger.
And my kids are normal.
But you know what???
My kids, are VERY wise, about people/vibes/and about "reading" people.
They are like your son.
Introverts at times but yet do like to socialize... but in smaller groups.
They both adjusted to school JUST fine. AND have plenty of friends and have NO socialization problems, at all. EVEN if they were like your son when younger.
Most kids are extroverts or gregarious.
Some are more inward or introverts or less gregarious.
BUT.... it does not make, one kid better than the other.
Meaning, extrovert kids are not better than your son.
I was also like that as a child... but I have always had, even as a child, a keener "sense" of people and situations and I was never a copy cat or a follower and I knew who I was. My kids are the same.
I am PROUD, of that, in my kids.
Be happy with your son.
Celebrate "him."
Teach him to KNOW himself, and to BE himself.
The more you compare him to others and make "excuses" for him to others, because you are self-conscious about it... the more it will give him hang-ups.
Your son has no "anxiety" problems.
He is a kid.
Only 4 years old.
MANY MANY MANY kids, are this way.
Like your son.
It does not mean he has psychological ills.