Rocky terrain? 4x4/pressure?

Is it wise to let pressures down . . . . Yep.

What to . . . depends on vehicle and tyres.

On rocky areas in my Patrol with BFG at's. Around 25psi , but may let more out depending (not normally past 20psi). In my Pajero with BFG at's , pretty similar.

Guys I know on MTr's run down around 15 psi on rocks , some a little lower , and the guys with swampers , run single figures.

Can you damage rims . . . well . . yeah, but its not normally as a result of airing down. Other factors , including where you're driving play a bigger part.

2. Part of the reason for having a 4wd is to make it as easy as possible. Thereby reducing the chances of damaging your vehicle, and chewing up tracks , so yeah, I would be using 4wd where ever possible (in H might be all it needs though).

I always lock the hubs in as soon as I go off the hard top you just never know whats around the corner and its a lot easier and safer than getting out of the 4x4 to lock them in if things go amiss99 GU Patrol 4.2 TD
Peter York 4x4

Letting your tyres down will make the ride more comfortable (as well as more grip if needed). You'll be amazed at the difference in ride if you drop the pressure. If you have standard radial tyres and you run them at around 32 psi normally try going down to 24 psi as a starting point. Drive sensibly and you'll be OK re damage to tyres and rims. Don't forget to pump them up again before any long run at higher speeds (eg back home on the blacktop). Speed and the resultant heat buildup will destroy the tyre.
I once hired a set of Baja Claws and ran them at 16psi offroad and they were really comfortable but they are designed to be run at low pressures - I wouldn't recommend that pressure for a standard radial in normal offorad driving.
There is no harm being in 4WD once offroad irrespective of what the track is like - fuel consumption may be a bit more but at least the components are getting lubricated. Whilst there may not be a need it is probably a wise thing.