In my personal opinion, a well trained horse doesn't lose their knowledge of riding when they are given time off, even if it was 4 years. They will be rusty, need a tune up possibly, but I see no reason why a well trained, broke horse would buck or run off, even after time off. They buck/run etc when they are scared or not trained well/not experienced. If they have been ridden before, I would assume that they are both trained and not scared.
BUT I wouldn't just hop on either. I would start them slowly and see where it goes, but I am the cautious type
So basically it does not surprise me that the horse let you sit on him, not at all. I don't think it hurt to slide off his back either. I would be cautious when riding, since you don't really know what issues he had under saddle, or how well broke he was when he stopped being ridden. When you do start riding, use a saddle for sure just in case.
So don't know if that helps, but I don't think you did anything wrong. Sounds like he remembers what its like to have someone on him, and that's a good sign.