I always went by the slip rule, if it feels like I'm occasionally losing traction I'll slip it into 4-hi since that can be done on the fly, also where you need to maintain speed like sand, mud, or winter conditions where you can use it for long periods of time. You don't want to use 4-hi on dry pavement, it needs some slippage. Most flat dirt roads 2wd is more than enough.
4-Lo for slow and steady crawling - not recommended over 20 or 25 mph or for long periods of time and not on dry pavement. slow and steady is in reference to crawling over obstacles where 4-hi doesn't provide enough torque, rock crawling etc..but always into 4-lo before or if i'm already into a situation and I feel I need it, I do it where I can come to a stop (keep your foot on the brakes), put it in neutral, shift to 4-lo then continue. It's also good for steep downhill speed control (steep loose dirt, gravel roads where there is some slippage), the lower range will help slow you down and you won't have to work your brakes so hard. Again, never on dry pavement or flat surfaces where you are traveling any distance, even though it's stated up to 20-25mph, I always come out of 4-Lo as soon as I'm done with the obstacle - complete stop, always in neutral 1st then shift in or out of 4-Lo, others may do it differently but I've never experienced any lockups or other issues.
I'm not familiar with the area you're going to but hopefully someone here is and will chime in.
Read your manual, it has good tips. Have fun!
