How can I minimize the cracking of a 4”-thick piece of elm wood I’m saving to make a table top?

That sounds like a great piece of wood. I’m glad you’re turning it into something good. Wood shrinks more around the circumference of the log than it does across the annual growth rings, and this is why wood cracks. The thicker a piece of wood is, the larger the portion of the log’s circumference you’ve got, the more internal stresses will build within the wood as drying occurs. And to make matters worse, I know from experience that elm is more prone to cracking as it dries than many other woods. You’ve made a good start by sealing the ends of the board soon after cutting because this is where rapid and unbalanced drying happens first. The key to minimal cracking is to slow the rate of drying as much as possible, and this means a long seasoning process in an unheated building. I’d give it 3 or 4 years. After that time cut the board to length, then measure the moisture content of the fresh ends with a moisture meter. If it’s down to 10% to 14%, then you can bring it into a heated spaced for further work. Even still, keep the temperature of your work space between 10ºC and 15ºC to promote slow drying. Some cracking is inevitable with wood this thick no matter what you do, but wax filler sticks offer one of the best ways to conceal cracks. Regular wood filler almost always makes a mess on projects like this because it comes loose in time. Instead, apply a finish to your completed table without filling any of the cracks that might be present. Bring the table into your home and let it sit for one full heating season. This will probably be enough to allow any inevitable additional cracks to appear before the wood stabilizes fully. Only then should you fill the cracks with wax. Choose a colour that’s identical to your table top or slightly darker. Filler that’s lighter than the surrounding wood looks bad.