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.
Breadboard ends were always confusing to me because they are trotted out as a solution for movement yet they solve two quite different problems.
On flush, jointed boards, they are a permanent jig to hold the ends in vertical alignment. Imagine taping your fingers together to keep your fingers flat. Lateral movement is impossible because the boards are glued tightly together.
If you’re concerned about lateral movement then the more important concern is to have gaps between the boards. The bread board end is now a rail in which your boards can slide like wobbly carriages on a train track: aligned in one direction (up/down for a table) but with the ability to move independently in another (across the width of the table.)
They aren't trotted out as a solution for movement. They are a solution to flatness that doesn't fuck up on the movement issue.