what do you need if you go 4 wheeling in the back country in California or one of the western States?

 First of all even in Northern California you can be 50 miles from the nearest person and 100 miles from the nearest bathroom or gas station and further from a restaurant. So, you have to be prepared for this by bringing your own food and water and maps. This can get even more extreme in places like Arizona, Nevada, Oregon, Washington, Idaho, Montana, Wyoming etc. etc. etc.

So, without physical maps (because your cell phone won't work) you either need a Satellite phone (very expensive) or physical maps

Why Forest Service physical maps? 

Because I think the forest service still makes these maps whereas most maps are found only on your phone now (cell phones smartphones). So, because of this you need physical maps because at 50 to 100 miles from the nearest person or deep in canyons or around large mountains there won't be ANY cell phone service unless you have a Satellite phone (And even then if a mountain or canyon is in the way of the satellite you need to bounce off of that won't work either (or if you forgot to charge up your Satellite phone).

So, you really need physical maps (even if you have a good GPS tracking device) in case you run out of battery charge for your GPS tracker, your smartphone or other device that might work in the bush around the world. It's true you might get solar chargers for these devices but remember it isn't sunny every day to do a solar recharge either. 

What happens if it rains or snows on you (as it does in places like Mt. Shasta above 7000 or 8000 feet even in August?

So, unless you are prepared for any eventuality you have to consider the possibility that you might not survive wherever you are going into (especially if you fall and break your ankle or worse).

So, going 4 wheeling in remote areas it's important to be prepared for anything (breakdowns, accidents, running out of gas, puncturing gas tanks on rocks too big to travel over or fallen trees or whatever.

So, whether you are backpacking or Kayaking or on Cross Country Skis or even split apart Snowboards that double as cross country skis always be prepared for anything.

Because if you aren't prepared one day might be your last.

Good luck this summer (and year round too).