For every other potion that can be "reversed" by a fermented spider eye, it is able to be first enhanced by redstone or glowstone, then (secondly) reversed by the fermented spider eye. But for a Slowness IV to be brewed it must be brewed from a regular speed potion, then fermented spider eye, then glowstone dust.

Every other potion can use glowstone first then fermented spider eye (or use it vise versa), but Slowness IV can only be brewed in the order of fermented spider eye then glowstone. Instead it will stop at Speed II and will not allow fermented spider eyes to be added to the potion. This make its brewing recipe different from every other potion of its kind.

Possible: Sugar>FermentedSpiderEye>Glowstone = SlownessIV

NotPossible: Sugar>Glowstone>\>FermentedSpiderEye =\= SlownessIV
(Although this should be possible.)