You’re gonna be just fine with the 3b+ Remember, the Norns is built originally off of the 3b, so you already have a bit more horsepower under the hood (many people have upgraded the computer modules in their factory Norns to a 3b+, but that is besides the point).
The desire for the 4 is just in potential future proofing. ORAC 2.0 is not as curated of an environment as the Norns, and its future is more at the whims of the market, so to speak. So, a 4 is potentially insurance on that.
But, let’s be frank here, it’s REALLY easy to swap the Pi’s on the board and if some monumental new step forward comes along, you can just pop off your 3b+ and slap on a 4!