Ultrasound and radar both spread out at an angle, and will be no good on a 4foot diam hole any deeper than 15 feet or so. Especially if there are lumps on the walls as they will give a strong reflection once the beam starts to spread.
Laser will be difficult but might work, if you can get it reflecting from the bottom. Normally if more than a few yards you need a reflector disk to get a good laser bounce signal.
As one possibly solution, drop something down the hole and measure the time until the "plop" sound is heard. You could drop something biodegradable like a sugar cube etc that won't hurt well water. If the device releases the object you should be able to measure time to a mS or better, and then compute distance.