Dear Mr. Ashdgee
Q1. "... I am putting together a datasheet for an MCC panel for ....,They require at least form 3b of internal segregation. I am listing form 3b for all the motors starters...."
A1. Reference IEC 60439-1 Edition 4.1 [superseded] , page 107 stipulated the "main criteria and the sub criteria for Form 3b and Form 4a. Attention: See also A3. below.
Q2. "... The MCC Panel has a very large AIR Circuit Breaker (4000A) occupying a full tier. For the incomer and large starters occupying full tier, can I specify form 4A ? I mean is it okay to mix form 3b and 4a or should I specify form 3b throughout ?..."
A2. A 4kA ACB may occupy a full tier to Form 4a. There are cases that the [assembly] may occupy only a part of the entire tier. It may ended up that a tier comprising of Form 4a and 3b... etc...
It maybe "advisable" to state e.g. [all ACB >1kA in Form 4a], [all MCCB >630A in Form 4a] and [all starters in Form 3b] etc.. The IEC 61439-1 [superseded] does NOT stipulate that the board shall be rated to the lower Form.
A3. History: The Form of internal separation initially appeared as the { x national appendix}. It was adopted into the IEC 60439-1 and the subsequent IEC 61439-1. This item does NOT appeared in the current IEC 61439-1 Edition 3.0 2020-05.
Che Kuan Yau (Singapore)

Why an MCC for service entrance? I never liked this idea...it's a pain to have to maintain/troubleshoot wearing 40 cal PPE. I guess if you can compartmentalize the main and have a "maintenance"/arc reduction switch...

Mike