So, the feat actually says this:
• As an action, you can spend one use of a healer’s kit to tend to a creature and restore 1d6 + 4 hit points to it, plus additional hit points equal to the creature’s maximum number of Hit Dice. The creature can’t regain hit points from this feat again until it finishes a short or long rest.
So, let's say I do this on a lvl 4 cleric. Would he heal 1d6+4+4 (because he has 4 Hit Dice) or 1d6+4+8 (because his maximum value of a hit die is 8)? Can anyone please explain it to me?