It is a total of 20% damage if you hit the target with all 4 elements (fire, arcane, cold, lightning) 5% per element used on the enemy for a damage increase for everyone in the group. It does not stack beyond 5 seconds either it will just continue to refresh the 5 seconds if you hit the same target with the same element.
For example: Let say you only have 3 of the 4 elements: Arcane, Fire, Cold this will = 15% damage and each of those 5% buffs will last 5 seconds as long as they are hit with the respective element within the 5 seconds.
So treat each element as a separate 5% buff:
Arcane = 5% damage increase for 5 seconds
Cold = 5% damage increase for 5 seconds
Fire = 5% damage increase for 5 seconds
Lightning = 5% damage increase for 5 seconds
Total =- 20% damage for 5 seconds per element