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.
Elemental Exposure. Damaging enemies with Arcane, Cold, Fire or Lightning will cause them to take 5% more damage from your attacks for 5 seconds. Each different damage type applies a stack, stacking up to 4 times. Elemental damage from your weapon contributes to Elemental Exposure.