Barring a move to an iPad version of MacOS, I can't see a logical reason for Apple to debut the M4 on iPad first over Macs.
Honestly, I’ve thought the same thing. One bold move would be to release the new iPad Pro with an M4 chip, 12GB of base RAM (hopefully), and the ability to turn into a macOS-like UI when paired to the new Magic Keyboard or plugged into an external display.
I’d prefer to have an hybrid option with a more traditional iPadOS UI on the iPad screen, and the more desktop-like UI on the external monitor. And… no, by desktop-like I don’t mean the current Stage Manager solution, but rather a more refined and pointer-oriented UI.
The price point of this device plus the keyboard and trackpad would definitely justify the canibalization of the lower end MacBooks.