How do I heat treat 4140 for hardy tools without cracking it?

Yes, oil is the right call, and it matters more the thicker the piece gets. Water on 4140 is how you get cracks, expecially on square corners and any sharp edges from the cutoff saw. Chamfer or round your corners before quenching and it'll save you grief. Heat it to just above the point where a magnet stops sticking, soak it there a bit so the middle catches up, then quench in oil. Warm oil, around the temp of a hot day, quenches less violently than cold oil and still hardens fine. A bucket of used motor oil is what most of us use, and keep a lid nearby because it will light up.

You are absolutely right about sticking to oil for these blocks; water is a huge risk when the sections are that big. For tempering those thicker pieces in a home setup, I quickly found the kitchen oven wasn't reliable on its own. I ended up building a small metal box and lined it with some ceramic refractory bricks to hold the heat much more evenly. It made a real difference in getting a consistent temperture throughout the part instead of just heating the outside quickly.

Don't stress about the middle not hardening all the way through. A hardy tool or drift doesn't need to be through-hardened anyway, a hard skin with a tougher core is actually what you want, less chance of it cracking in use. Normalizing's the easier fix too, just pull it out of the fire and let it sit on the air instead of leaving it in the coals. Cooling in the fire makes it go soft and gummy, wich is the opposite of what you want before forging the next session. And on the kitchen oven, grab a cheap oven thermometer if you can, the dial on mine was off by a good 75 degrees till I checked it.