Approximately three of them would be roughly equivalent to four book skeletons, so CR 1/2 rather than 1/4.
They'd carry dwarf weapons like the mace, battle axe, warhammer, doing 1d8 +3 instead of 1d6 + 2 damage.
I'd maybe do dwarf skeletons with 16 CON and 12 STR, that wear more complete armor (AC 14 or 15). Just be careful not to imbalance things or pose a quandry the party isn't capable of solving or surviving. It's always good to give your monsters some twists that players won't know from reading the MM. (Assuming a homebrew campaign) You are the DM. Monstrous Compendium Vol 3: Minecraft Creatures
