Honestly, spec for spec, I am staying with my Fold 3. I just can't see enough of an upgrade here for me to jump over. However, this does not mean it's not a worthy upgrade for others.
This is the curse of the Even releases.
Fold 1 - innovative, first one, shock and awe and VERY Bleeding Edge
Fold 2 - Fixes many lessons learned from Fold 1. Typical of a second gen release
Fold 3 - FINALLY got it mostly right. Especially the water resistance and the inner screen. Upgraded camera's and everything else. Worthy of an upgrade
Fold 4 - Fixes even more things from previous generation (mostly unnoticed tweaks and all that), upgrades to hardware, but nothing that is a generation leap forward
If the pattern holds, the Fold 5 will be something big.
Of course, this is all supposition and speculation, nothing concreate to support this, just my own personal view.