[Nick_K]

AFAIK here's how attacks work:

If a single weapon has multiple attacks all of these are directed at the same unit. This means that the gorgon cannot kill more than 1 unit each action with its snake hair, and the hydra cannot kill more than 1 with each of its bite attacks.

A unit with multiple weapons can use them on different units, but can only attack a maximum of 1 square. This means that if each square contains 1 unit only (e.g. jotuns) then even the most powerful SC or doom horror can kill at most 1 unit per turn with weapon attacks.
However, units only make 1 attack animation in total for all their weapon attacks. This is probably why you thought the wyrm was not attacking 4 times. Actually each attack animation represents two attacks.

Each unit has a number of 'actions'. This is usually 1, but it can be 2 if they have quickness or even higher with heroic quickness.
Each action counts as an extra turn in melee combat. A quickened unit can attack two squares - one with each action.