When I said will you give me 4G sim he said sir currently only prepaid new sims are being issued under 4G. This postpaid sim is 3G but you can replace it after activation if you need 4G.
I suspect this is what happened with me as well - when I got the SIM, it said Jazz (which are presumably the 4G ones).
When I converted from prepaid to postpaid, I got a SIM that said Mobilink (which are presumably the 3G ones).
And yesterday, I got the 4G one (which, again, says Jazz).
I think this cycle is unintentional - probably because I didn't ask them to give me a 4G SIM (and at that time, there was also no news on the 4G launch).
So to sum it up, if the helpline is telling you that 4G has been activated against your number and that your SIM is also 4G, but you do not receive any 4G signals, then your SIM is NOT 4G-compatible, regardless of what they tell you.
As I said, I only found out when I went to their service center my SIM wasn't 4G compatible because it showed on their screen 4G was deactivated due to the wrong SIM type.
Edit: typo