I've been thinking about your post, and I've had a look at your pics. Your horse looks a nice lad and you look good on him. However, as you've asked about bitting your horse, and you have concerns that he may overpower you if he decided to take off, I would like to mention a couple of things which worry me.
Lots of people say they have problems with heavy cobs when they reach six/seven years, when some of them seem to go through a rebellious period and give cause for concern. Heavy horses and their part-breds can take a long time to mature, both physically and mentally, and I don't think a rising 4yr old of that type can be considered 'fully broken'.
Also, because the growth rate is slow in such horses, I would worry that a heavy horse may suffer damage to his legs if ridden too much at that age. There is the potential for stresses on the growth plates of the long bones at certain times during the horse's development and, in extreme cases, the longevity of the horses working life may be decreased by too much work at too young an age.
I am concerned that your horse seems to be coming behind the bit in the pic you posted. I acknowledge that it is only a brief snapshot in time, but he does look held a bit tight. I wonder if that is because, as you have mentioned, you are slightly afraid that he may have the power to run away with you, and that you may not be able to stop him.
Snaffle bits are not as kind on the horse's mouth as many people believe. The single jointed snaffle in particular has a nutcracker action which puts pressure on the bars of the mouth, and can cause the joint to put severe pressure on the roof of the horse's mouth if used harshly. Some riders, under the misconception that snaffle bits are mild, can cause pain to the horse's mouth with very little pressure from the hands.
In reaction to such pain, some horses will throw there head up and try to lose the pain by getting 'above the bit', while others will try to avoid pain by bringing their chin as far back toward their chest as they can (coming behind the bit). Some riders even think this latter is normal and that their horse has been 'on the bit', particularly if he is foamy at his mouth, when really the horse has his head so far behind the vertical that he cannot swallow effectively.
Your enquiry about a more appropriate bit, in case your horse should try to run away, and your remark that he may be too strong to stop, tells me that neither you or the horse feel that you fully trust each other yet. A green horse can still be a safe horse, as long as he has been trained in such a way that he understands that he cannot use his strength against you.
If the young horse has learned that his strength is greater than that of his handler, he will always be a liability if he decides not to do as he's asked. Regardless of the bit in his mouth, his experience will tell him that his strength will let him win. If the rider attempts to hold him with the bit and, particularly if that causes him pain, the horse may try to run through the bit and the rider will struggle to stop him. Some horses will be difficult to stop and, if held tight, may resort to rearing.
With green horses, I would always recommend a mullen mouthed bit, preferably with full cheek. Such a bit, correctly fiited with Fulmer loops to the cheek straps on the bridle, will always sit well in the mouth, and will present an even pressure on both corners of the mouth, without affecting the palate. The Pee Wee bit is another which I have found to be liked by young horses. It has a light curb action, a mullen mouth piece, and encourages a horse to yield in the turns. With made horses, we have found it to be a great help in creating a balanced, responsive horse which can be ridden with a light contact.
Any bit is only as good as the hands which use it. Pressure and release is very important when using a bit of any sort. Much depends on how the horse was introduced to it, and I would always recommend some groundwork with a new bit, where the horse is trained into the bit before being mounted. The most important lesson a horse will ever learn is how to yield to pressure, and that doing so will earn him relief from it. A horse trained thus will always be keen to comply with the rider/handler's requests as he will have learned from experience that any pressure brought to bear on him will be released immediately he does.
I don't know how you ride, but I do know that some riders rely very heavily on the bit to be able to apply the brakes. In fact, much can be done in other areas of the horse's training which will make him more compliant and trustworthy, and will make the use of the bit an absolute last resort when it comes to stopping him.
I firmly believe that groundwork is the method by which a trusting relationship of mutual respect is established between horse and handler and then rider. A partnership takes a while to build, and is formed over a length of time which is dictated by the learning ability of both parties. Teach your horse that there are other reasons forbye the bit that he must not run away from you.