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US did not have anything heavier as a tank than the M4 until the M26 came along in 1945.
Not entirely correct, the US standardisd the M6 Heavy tank in 1942?43? and made a number of them (200ish springs to mind, but I don't ahve references handy). They were not used in combat and didn't leave the USA.
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The tank destroyers were designed to fight other tanks.
Again, close, but not quite right. The tank destroyers were not supposed to fight with enemy tanks, but to "aggressively" kill them without going "toe-to-toe". The TDs never had the armour to stand up to even light tanks. They were supposed to maneuver as large batallion, or even brigade-sized ,units to the flanks of enemy armour breakthroughs and kill the enemy tanks from enfilade positions. I'm pretty sure that this was never ever actually done in the field as the conditions never presented themselves as the creaters of the doctrine had imagined. TDs were certainly never employed as battalion sized units in their intended role.
Paul