What? I see you are from Ontario and maybe this is good advice for your flat driving conditions, but I can assure you on the mountain passes here, where they film Highway Thru Hell, having 4wd on the highway is a heck of a lot safer. I pass many 2wd trucks and cars on the highways, spun out in ditches on the hills.
OP - I have had my 4wd system go nuts switching out of 4LO while still moving, so I always stop, put the truck in neutral, then switch out of 4LO. However, I switch in and out of 4HI at speeds all the time.
4wd might help you get moving forward, but that's it. 4wd does not give you better braking or better steering.
Just because you passed someone who spun out, that doesn't mean you have more traction than them. If you take 2 identical trucks, with identical tires, and drive on in 4wd and one in 2wd,they both have the same cornering and braking ability.
The best way I can think of to describe it is that a tire has 100 units of traction. When you're accelerating in a straight line, all 100 units of traction are being used to go forward. Same thing when you brake in a straight line. Also, if you're turning left or right, you can can use all 100 units of traction to turn.
This all changes when you're doing more than one thing at a time. Turn left and press the gas? Now you have 30 units of traction turning you and 70 units of traction pushing you forward. Essentially, you can no longer accelerate as quickly, or turn as sharply, before losing control.
If you are cruising along the highway at 100km/h, you might have 10 units of traction pushing you forward and 90 units of traction for steering, but anytime that you exceed 100 units of traction, you will lose control.
So, turn too sharp, push the gas or brake too hard, and suddenly you're asking for 200 units of traction, but the tires can only give 100.
4wd doesn't change this. At low speeds, yes it can help you get moving forward, but that's it. You're still just as likely to spin out as the guy in the 2wd truck, only you might make it further up the hill than he can.
I did a weekend racing school a few years back, and that instructor explained it all better than I can. But watch Forumula 1 on tv. Those guys do ALL of their braking before they turn, so that they can use all of the available traction for the corner. This means that they can take a corner at 100km/h instead of 80km/h if they were touching the brakes during the corner.