The 2mm chassis works much better than the 3mm under most circumstances. Trust me I have done a ton of testing with both prototype pro 4’s and the pro 3 on all types of track surfaces and tire combinations…even foam.
Kamen Rider,
Hara has played a huge part in the development and testing of the Pro 4. If he decides that the 2mm chassis works better, you can bet its going to be in the kit.