The sub-PANEL can have as high a rating as you want as long as it is more than the wire feeding it and the correct size breaker in the main panel protecting that wire. From a practical standpoint, you can do a 100 amp sub panel off of a 100 amp main load center. You would use a 100 amp breaker on the sub panel feeders in the main panel. BUT, a #4 CU wire would only be rated for 90 amps for a sub panel feeder off a main load center, so you would either have to use a larger wire guage with the 100 amp feeder breaker, or a smaller breaker.

The reality is that you need to do a load calculation for what the sub panel will feed, and size everything accordingly. Unless you are running this wire to another building, the sub panel doesn't need a "(main)".

90 C copper wiring (if that is what is being used) is good to 95A and the overcurrent "trade size" rule probably permits a 100A breaker to be used.

Further, while I know what Fred means, what he said is technically incorrect. You must put a breaker in the main panel that protects the feeder to the subpanel. You could put a 400A subpanel in on 12g wire as long as you protect it with a 20A breaker in the main panel. The wiring itself means nothing with regard to the subpanel ampacity (or the breaker in it).

You need to use the 75°C column when sizing the wire, which makes #4 CU good for 85 amps. according to the breaker manufacturer specs.

I thought that is what I said? Whether you chose the breaker first or the wire, they both have to work together and you end up with the same thing unless you were to deliberately size the breaker smaller than necessary.