As I understand it 4wd_only is not a typical access tag. I believe 4wd_only=yes specifies that motor vehicles lacking 4-wheel drive are prohibited. This represents signage warning of a legal restriction. If you drive a vehicle that is not equipped for off road use past these signs there are legal penalties, especially if you get stuck. On the other hand, 4wd_only=recommended is for cases where the signs just specify a recommendation or warning, not a legally enforceable restriction.
I think the reason this is not a normal access tag despite the legal penalties is that you can’t always tell just from looking at a vehicle if it is “properly equipped for off-road” use. The same model of pickup truck is often sold with and without 4-wheel drive. If you drove the non-4wd version past a 4wd only restriction sign and didn’t get stuck, it wouldn’t be obvious that your vehicle is not allowed. If you got stuck and called for help, then it would become clear and you’d be penalized. This is fairly different than the access tagging vehicle classes (motorcar, hgv, agricultural, etc) that are easier to distinguish at a glance.