I've only ever seen a single 4 Slot Shadow Wyrm. It was already tamed by someone and had Armor Ignore added to it, but that was on TC before they allowed them to have full HP after tame (they used to lose half their max Health after tame). I don't think they can end up as 4 Slots anymore, now that they tame with full Health. Between 900-1,030 STR (high stat weight, WAY overcap), 560-600 Health (high stat weight), 485-620 INT, as well as their Magery and Necro, i don't think it's even remotely possible for them to spawn as 4 slot now.
Pity too, because their up to 1,030 STR, and 29-35 Base Damage would make them great damage dealers, if only they could reach 120 Wrest. Haven't tamed one on live shards yet, but on TC they would also cast Corpse Skin, which would lower their melee damage output on the target.