Can a Rebel Hero use the healing mechanic when they haven't suffered any Strain to regain Hit Points equal to their Endurance?

So, Fenn has 0 strain on him, but he has 4 damage. Can he use an activation to heal 4 damage (4 being his Endurance rating) even though he has 0 strain?

Absolutely! They can, and should!

In addition to this the strain rules clarify that if you could ever heal strain and have no strain to heal you may instead heal damage.

Page 23 - Strain in Campaign.

Fenn can also use this rule to heal damage with his Lone Wolf ability.