If you want to race competitvely, reduce your lap times and know how to find a good set-up for the surface you are running on, all these features will bring you a great step or even leap forward. Even if the car should not feel any faster at first, lap times don't lie. If you switch from an old-school XX to a mid motor car and run on a track with a decent amount on grip, you'll notice the difference instantly. Switching from a B5M to a YZ-2 for me was almost as big an improvement as converting my 1st gen B-Max2 to the MR version.
To sum things up: Properly designed laydown cars are a noticable improvement over std mid-motor cars, and all the features of the 4.0 improve adjustability and address the few design shortcomings the 3.0 used to have. I doubt that you'd be disappointed if you'd give the 4.0 (or any other big manufacturer's current 2WD) a try. Also, the current breed of cars is much more durable than anything before the original TLR 22, so it would be kind of you to send your classic XX into its well deserved retirement.
