4 years is more than enough time for drying. Rule of thumb is year per inch of thickness.

I'm just speculating here but probably the support structure didn't allow for wood movement. You need something to keep the table top flat while allowing it to move. Screwing it to a stiff frame (steel or cross grain wodo) is certain to crack when the wood moves.

Breadboard ends, sliding dovetails or steel support with elongated holes (going to a threaded insert and bolt) are good ways to support a table top.

The wood was probably stabilized to your shop atmosphere but indoors in the dry winter, maybe with air conditioning or a fire place, and there's going to be movement.

Kiln drying does not stop seasonal wood movement.

If you share a picture we can take an educated guess what caused the table to warp and crack.