Had some poplar milled from some large trees we had to take down here. Air dried in my shop for 4 years before having it made into a table. All it took was 1 winter and it split and bent severely inside the house. I will only kiln dry from now on.
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.
Four years of air drying may not have been enough, depending on the thickness of the boards and the moisture level in the air. Also the issues of wood movement and grain direction must be considered during the design and manufacturing of furniture with that wood. Home sawn wood will often have knots, randomly curved grain, etc, so it can be more difficult to get predictable results.
I dried three red oak trees using a dehumidifier kiln. ( 4'x4'x16' 1" pink insulation foam box assembled with packing tape with a household dehumidifier and fan inside. Very low tech. Knock it down when not using it.)
The process is mostly: measure moisture content of wood, pick a humidity to maintain, check wood periodically to see if it is drying too fast or too slow. Weigh water coming out to monitor process.
Very low effort if you have space to allocate while in use. The wood came out well, no complaints.
One downside is you won't kill insects with heat, so you could have trouble if it is buggy wood.
I’ve never dried anything that long indoors, but from what people told me when I was researching the best way to dry some red oak I had milled there are issues drying indoors doors. Wind does most of the drying outside.
Did you use a moisture meter?
Worse, I've been told that attempting to dry indoors would result in rot (given not enough air flow, which might not be the case in all indoor environments), and consequently have never tried it. I've exclusively air dried pine, oak, poplar, pecan, cedar, all under open walled structures and not had too many problems over the last 20 years (and my dad was doing it for another 30 before that).
Poplar is a very wet wood. It tends to take so long to dry, and then burns so quickly, that it isn't worth processing for firewood!
What was the moisture % at the end, and what was the joinery of the end product?