At what gradient does an climbing bike over takes an aero bike? And would an all arounder be preferred in a weird sweet spot like 3-4%?
Depends on how fast you are going and how long the climb is. 3-4% should still be in aero territory for almost all categories.
Sustained >6% is probably where I’d assume it starts to matter ASSUMING: there is not a descent; at which point it no longer matters and aero will always be the better choice.
Honestly the only time I think a climbing bike is preferred is when trying to go for a PR on a single climb segment. The time in races is made up for in draft… yes, even the draft you have in segmental portions of ADZ.
In my humble opinion only at Alpe du Zwift, Ven Top and Ven 10. And perhaps the Radio Tower and the Grade, but only if you are just doing those. If they are part of another route I would not bother with a climbing bike.
In all other routes the aero frames will be more beneficial and you can always play a bit with different wheels.