The safety of 4-wheelers has greatly improved as far as the design of the 4-wheelers themselves over the past few years with manufacturers listening to their customer's concerns and addressing them in the design of the newer models of 4-wheelers.
Many newer models come with standard safety features such as hand guards that protect the 4-wheeler rider's hands in ways that gloves cannot. Debris and branches can loosen grip on the handlebars, with hand guards molded out of heavy plastic, there is nothing getting to the rider's hands. Therefore, good grip and safe maneuvering of the 4-wheeler can be maintained.
Some essential equipment you should never ride your 4-wheeler without is of course starting with your DOT approved helmet. Never operate a 4-wheeler without a helmet on your head. There is no point in risking brain injury or even death because you think helmets are too heavy, not cool or you just do not wish to wear one. None of those are good reasons to not be safe when riding your 4-wheeler.
Eye protection is a big concern. Full visor helmets are a good start for protecting your eyes from flying debris and other objects when riding your 4-wheeler but it is just that, a good start. What you really need to wear when you are riding your 4-wheeler is a good pair of goggles. They keep flying debris out of your eyes and allow for better vision, by blocking the wind and other distractions from your eyes and line of sight.
Gloves are a must. Even if you are lucky enough to own a newer model 4-wheelers that has the hand guard, always wear your gloves. They are to help you keep your hands from getting injured from debris, branches and such when you're riding your 4-wheeler but also their purpose is to help you maintain your grip. If you use your gloves when you ride, especially one of the newer generation of pre-curved finger gloves, you will experience less fatigue. Boots protect your ankles and shins from branches and other objects, and keep your feet dry. A jacket is another must have as it protects your arms and body against cuts, bruises and scratches caused by tree limbs, rocks, etc. Long pants protects your legs against engine and exhaust burns, also protects your legs against cuts, bruises and scratches by rocks and limbs.
You don't need a lot of safety gear to ride your 4-wheeler but the essentials will make your ride a lot safer and more fun!