spells – Is Flame Wisp’s damage added to the Strike’s damage or dealt as a separate damage instance?

Flame Wisp deals damage when you hit with a Strike:

Three faintly glowing wisps of fire float around your head. Each time you hit a creature with a Strike, one of the wisps goes hurtling towards that creature, dealing 1d4 fire damage.

Is the damage added to the Strike’s damage or is it dealt as a separate damage instance?