Horspoor just asked a lot of questions I had. First, 4 is super young for an Arab. I don't even start breaking my horses until they are 4 (sometimes 5) due to their immaturity and how they handle stress. Most at this age are NOT predictable no matter how great of a horse they are.

What are you feeding him? High protein grains or legume hay is a big no no for Arabs unless you are ready to ride a bucking bronco on a regular basis image Again, how much turnout does he get? If he is in a stall most of the time that can also factor in his energy level. Do you lunge him before getting on? How long have you been riding and what is your training experience?

What I would do with this guy if he was in my care is start over. I would lunge him daily and gradually introduce side reins. Once he is lunging nicely both directions with side reins and knows voice commands for walk, trot, canter, ho, I would then start riding him. I would always lunge him first with side reins and then have someone hold him while I mount so he cannot bolt and take off. I would then ride him on the lunge line so there is a ground person that also has some control. I would gradually increase his riding time and get him off the lunge line. This can literally take months to progress. Some Arabs do not have the mind to move quickly, slow and steady is my motto.

If this all failed then I would consider him a bad egg but not after I tried all of this. Is there anyone (like your sister) that could help you do this or are you too scared now? (which is completely justifiable). I wish you were closer so I could help