We have 4 wheel drive vehicles with Drive plus a lower 1 and 2 gears. Our small hill is icy. My husband insists on going down in neutral [he is 68 and I suspect ‘daddy’ did it that way on the farm] I say you are safer with it in gear. I am 72 and we have been on this topic for 45 years-- so I really think this argument has gone on long enough! 
IN GEAR IN 4x4.
When in 4x4, even the slightest engine braking ( foot off the gas ), and even a tap on the brake pedal, will be applied to all four wheels.
If in neutral you have ONLY your wheel brakes to attempt to slow you and no recourse ( except to pump )if they lock up.
Toooo long! I was a driver/mechanic in the army and would have been disciplined for going down hill in neutral.
You have much better control in low gear than in neutral.
A small icy hill needs a dose of snow melting salt and some grit.I use kitty litter in bulk from Walmart.
In gear, but I suspect that knowledge being given to you is not going to change his habits of beliefs one bit.
It’s MUCH harder to “lock up a wheel” and skid when IN GEAR!! When in 4wd, all the wheels are connected together and braking force evenly distributed to all four wheels EVEN THOUGH ONE BRAKE MAY BE TRYING TO LOCK… If it’s THAT slick, stay home!
You paid good money for 4wd. Why throw it away by coasting down a slick hill?? 4wd works when BRAKING too!!
When I’m going down a slippery hill in a 4WD vehicle I put the transmission in low gear and let the engine and the 4WD retard my forward speed. I use the ABS brakes if necessary.
Neutral might have advantages with a 2WD vehicle (especially RWD), but not with a 4WD vehicle.
Good luck convincing hubby, however. At this point he may be doing it just to annoy you.
A dope slap seem appropriate here.
I go along with the in gear with 4WD.
Other than that I would suggest that someone with lots of experience, especially with the car in question may be able to do better in neutral, but for most people it will be better in gear.
Note: I expect there to be differences with 2 wheel vs 4 wheel drive, and auto vs manual as well as many other factors.
That said, some of the modern computer controlled systems can do even better.
We really need professional help on this one. Growing up in icy snowy roads and learning the hard way neutral was always the safest position in my experience. Maybe abs adds something and pumping the pedal is the predecessor to abs. I do not know where to check the results, so I guess the best analysis is who is in the ditch and who is on the road. Can you offer any experience?
On a snowy driveway I guess in gear in an automatic might be OK, but on real ice or hard packed snow I always went with neutral. In a standard shift you’ve got the clutch, so the question is completely different.
Especially on a cold morning the fast idling engine, in drive, kind of urged the car down the hill and made it very difficult to keep things in line. My driveway was in a field, downhill from the house, facing south, so it would often get very icy overnight after a bit of melting during the day from the sun. Sometimes sliding off, into the snowbank very slowly, was how I inched down to the road. In the afternoon the warmed softer snow or ice gave me enough grip to just get up, sideways, back and forth, whatever as long as it was moving toward the house it was OK with me.
If this 68 year old guy has been getting down the driveway in neutral for 45 years, and he knows how to make it work, then let him do it.
AS for me, I moved to California.