Some of the most important gear you should bring on any 4x4 trip is a good first aid kit. I'm not talking about a handful of band-aids; I'm talking about a kit that can help you and your companions survive a long, slow trip back to civilization when an injury happens in the backcountry.
Expedition Portal has a well-written article about first aid kit considerations. I agree with their suggestion to build your own first aid kit that meets your needs rather than simply buying a generic preassembled kit, but I think it's best to start with a decent kit from a company like Adventure Medical Kits and customize yours from there.
I like to have a very complete kit that contains not just basic first aid medical supplies but also some supplies for more severe injuries as well as basic survival and search & rescue tools. I prefer to call my enhanced first aid kit an Emergency Kit.
I started out with an Adventure Medical Kits Sportsman First Aid Kit which works well for 4wheeling and backcountry camping because it is designed with puncture wounds in mind. In AMK's own words, "Designed for recreational hunters and fishermen on trips up to four days long, the Sportsman contains supplies to treat the most common injuries, including penetration wounds from bullets or arrows, fish hook removal, stabilizing sprains, and stopping severe bleeding."
It's a very compact kit that includes basic tools and supplies as well as A Comprehensive Guide to Wilderness & Travel Medicine by Eric. A. Weiss which can help an inexperienced person correctly perform some pretty impressive first aid measures:
Any multi-day first aid kit should include a number of tampons, which are good not only for their intended purpose but also as a sterile insert for a puncture wound from a knife, tool, or bullet. I also have a small 2-segment pill holder with ibuprofen and vicodin pain relievers.
On my various offroad adventures over the years, I've seen some pretty nasty slip-and-fall's and tool injuries, so I added the QuickClot Sport and an Israeli Battle Dressing Compression Bandage to my kit:
In the event of vehicle breakdown or some other event that leaves us stranded overnight or in the cold, I've added a pack of 10 mylar emergency blankets:
Water is another critical element to survival. I always carry a 5-gallon bladder of drinking water in addition to bottled water in my cooler, but in the event of an extended time stranded away from a safe source of water, I included some Katadyn Micropur MP1 water purification tablets and an Aquamira Frontier Emergency Water Filter to my emergency kit:
The ability to see and be seen is also critical in an emergency. A Search & Rescue crew can't find us if we aren't visible, so in addition to a small LED flashlight with spare batteries, I've included at 10-pack of Cyalume SnapLight Industrial Grade Chemical Light Sticks.
I added some other survival gear from an earlier collection, including a road flare, a couple boxes of matches, a cigarette lighter, a magnesium firestarter, some easy-light firestarter gel, a handheld strobe light, a whistle, and a multitool.
To contain all these items and keep them handy no matter where we go and what vehicle we use, I spent $5 at Harbor Freight Tools for a 12-inch tool bag. It's just the right size and is made from a heavy-duty canvas in high-visibility yellow.
If there's anything you think we might be missing from our Emergency Kit, feel free to leave a comment below. And don't forget that medicines and other first aid supplies do have a shelf life - check the expiration dates in your own First Aid/Emergency Kit to make sure you will have what you need if you ever wind up in an emergency.