When is it a compulsory call ?

[illegit]

This is far from a mandatory call. First off in this situation you have FOUR live players to act after you who could re-raise meaning you won't actually end up getting the odds you thought you were getting. Secondly 4-1 odds are significant odds heads-up or 3-way, but in a 5-6 way pot you're getting 4-1 in a spot where your equity in the pot could easily be, and almost certainly is, less than that.

If you're getting big odds like this AND can close the action, and with deep enough stacks then you should definitely call with anything even semi decent.