If I allow people to ride off-road 4-wheel vehicies on my property and someone has an accident, can I be held liable or be sued?

Whether you could be held liable or be sued are two different questions. Can you be sued - yes you can since you can be sued for just about anything including the color of your pants. Can you be held liable is the question that has the real legal issues. Whether you could be held liable, i.e. at fault or held responsible to pay for any harm / injury suffered would depend upon many factors not stated in your factual statement. Merely allowing people to ride on your property is not in and of itself a reason to find you to be at fault or held responsible unless some other unstated facts are subject to consideration. For example if the cause of the accident was some hazardous condition inherent in your property and you know about the condition and did nothing to warn other then yes you could be held liable. If on the other hand someone drove into a tree and is injured then no you most likely would not be held liable. There are many factors that an attorney would have to consider in accessing a suit against you thus given the facts as stated you should visit with an attorney to more fully discuss the matter.