Sixcolors has a great article on this very thing. It seems like TSMC's M3 process was a dead-end with too many failed dies. M4 is easier to make and has better implications for Apple's AI plans going forward.

If what they said about it performing at M2 levels with half the wattage are true it may mean that we are going to see Macbooks with astronomical battery life.

And if Apple can make a 5mm iPad Pro they may well also built a 5mm Tandem OLED Pro Display or even a 5mm iMac. How about a 20mm Mac Mini?

The implications of the M4 on Apple's future product designs may well give us some very interesting tech going forward.