4 speed Autolite HiPo carbs "should " be in less demand as they ( almost) never were used on a GT350. The automatic version has become very expensive because it was and YES by all means make sure it's not a CLONE. Some misinformed still associate the 4 speed carb with a GT350 and the only possible use would have been a FACTORY supercharged 4 speed '66. The registry has those cars noted.
Randy
Actually Randy, the 4100 Autolite 4-sp Hipo carbs are in more demand. Because they were factory installed in most all K code Mustangs. And of course, Lol, they were more K code Mustangs than GT350's, therefore making them MORE in demand.