Personally, I don't think you should hitch up a 4-in-hand without at least one of them being older and solid.
I have always trained our hitches with an older horse. (learned that lesson the hard way), and always train them to all sides and all spots.
I have to laugh though - when I was younger - I would get on anything, anytime, anyway.
Then this strange thing happened - it started hurting a whole lot more when I hit gravel, or dirt, or rocks, or pavement.
You mentioned something though - holding on.
I have always had that problem - no matter what is going on - I hang on - which is not always a smart thing to do. I think its my dad's fault, he would always say "Don't let go" - then, as I grew up - I realized I couldn't let go - unless I made a concious effort to let go. That - will get you dragged, stepped on and other unpleasant things.
I remember training one girl to cart - by herself - she was coming along perfectly. Then, I had a holdback break - and, he bumped her in the butt.
She went ballistic - all I saw was head, hooves, head, hooves, head, hooves, and rapidly picking up speed. I did the "chariot thing" too. I really wanted to bail, but I figured I could stand on the seat and lean my entire body weight into her, and maybe get her to turn - or at least slow down.
She didn't, and Roy, who was running behind, yelling "jump, jump", so, I just jumped straight up, and let go, and she pulled the cart out from under me, and I landed, rolling onto the dirt road. She continued for about 3/4ths of a mile, got the cart turned over, got away from it, and when we caught up to her, she was beating the living daylights out of it, kicking it for all she was worth.
She never forgot that spot. Forever more, even though she eventually became a completely trained horse, when we got to that area, you had to hold on, because she would shy at that spot.
The worst "bucking off" I ever got was from a Belgian mare - man, you have NOT been bucked, until you have been bucked by something 18 hands tall - she was a big son-of-a-gun, never had bucked before, but that day - ------ I wonder what gets into their minds, sometimes. And, it is NOT easy to stay on something that has a back so wide that your legs are barely hanging down. And turning the head of a Belgian is NOT that easy, either. In fact, we had one horse (not a Belgian), that I think was trying to kill me. I was training a palomino for a friend, and, he was not real familiar with horses, and had asked me to do a particular thing (cannot remember what). Anyway, the witch decided to run away with me, and was headed straight for the barn, by way of the clotheslines. I tried to turn her - at a fun gallop, and couldn't, so I leaned over and grabbed the bridle itself, and turned her head right onto my knee. Now I had a horse who was not only NOT turning, but running straight at full throttle, with no sight - she never slowed down, nor turned one little bit. The only thing left to do so that I wasn't decapitated was lay down on top of her. She went under the clothesline, through the barn fence, and smashed into the barn door, and knocked herself back on her haunches, me still in the saddle. I yelled to the guy to open the gate, as, after all that, I was NOT gonna to let her win. And I forced her to run back out the gate, and then walk to the barn, and then when she started speeding up, I circled her butt for about 15 minutes, until she gave up and decided to walk. Then I walked her back away from the barn, got off, and led her to the barn (she needed some drying off, since she was real sweaty). Lucily, that was the only time she ever did that - if I had let her get away with that, her inexperienced owned would have probably gotten killed eventually.
Why do people who are not real familiar with horses, or not experienced riders, buy horses that are green-broke? I see that all the time.
Their first horse - or first AND second, should be sound, willing, older and well-broke.
Beth
Nothing suggests a simple vertical sinking, but on the contrary a controlled movement. 
