I regret having bought my husband a white gold ring (18K). Within a year, the rhodium plating had come off in some parts. He's rough on rings as he's not used to wearing jewelry.
My husband's wedding ring is made of tungsten carbide and he loves it. I think it's a 6 mm. band, and it's a size 10. 4 mm. would be pretty thin. If you husband's hands are small, it might be ok. Otherwise, I'd get at least a 5 mm. band.
On men I like heavy comfort fit and about 6-7mm on a size 10 hand FWIW.
My hubby wears white gold and it wears well. He likes the patina it gets as the rhodium wears off. A simple polish and dip and its like new. We opted for 10k so it was whiter. You can also get one of the new alloys that does not need rhodium.
I'll be getting him a new ring for our fifth anniversary this year and it will either be a titanium tension set ring or gold with smaller accent stones.
My DH picked a 6mm band, he was originally looking at 4mm but he decided it was too thin on his finger (size10 as well).
I'd suggest you ask your DH what he'd prefer. Even if he's never voiced an opinion on your jewellery he might surprise you by being quite picky about what *he* wants to wear!