Note that the 4-12-2 firebox was above some of the drivers. GN did this, too, on all their articulateds.
Some thought it necessary to have a tall capacious firebox, and so used a trailing truck that was substantially lower than the drivers. Somehow, the 2-6-6-6 comes to mind.
Both approaches seem to have worked pretty well for their respective railroads.
The big difference is that when you have a trailing truck, it takes weight off the drivers. And the bigger the trailing truck, the less weight on the drivers.
The one version quite possibly could generate more steam so the locomotive could go faster. The other pulled more.
I'm not sure of the timing, but I do wonder if the 4-10-4 wasn't a move to make the 4-10-2 more of a passenger engine, which service it did perform. I note that the 4-10-2 did not have its firebox over any drivers. If you wanted more steam out of that engine so as to go faster, a 4-wheel trailer under a lengthed firebox is reasonable. So is enlarging the drivers.
And, of course, if you enlarge the drivers, you interfere more with the firebox, if it happens to extend over the drivers.
I believe the locomotive COULD have been successful.
Ed
