Yeah - I had that wrong - I was thinking the angles between cylinder banks... but that wouldn't make an x, either (that would have just 4 cylinders)... a 6 would be more like a an asterisk (not the * that I see on the screen when I type, but the six-armed one I have on my keyboard)...
an X-4 would still have some terrible vibrational modes - either the pistons all move to the top half of the x together and then to the bottom half, or they move left-right together (if any of that makes any sense). In any case, you have a lot of reciprocating mass moving in the same direction without a counterbalance. Now maybe you can counterbalance the shaft the cam lobes are on and fix it..... But I doubt an X-4 would be any better than a flat-4...
Good point. The piston pairs would have to be run in sets of twos.
Your post reminded me of a friend of mine who lost both buttocks in a shark attack.
He still dives, and doesn't bother to get out of the water when there are sharks around.
He figures he doesn't have an asterisk...