Can Janna heal back damage from --4 cards?

Janna can heal back damage that has already been marked on your units, but there's an important timing catch.

Janna's "When you play me" trigger goes on the chain after she enters — so if you play her as a Reaction to a spell that's about to deal damage (like Void Seeker dealing 4, or Falling Star dealing 3+3), her heal resolves first (LIFO), and then the damage resolves second. She heals the damage before it happens, so she cannot prevent that incoming damage from the spell.

However, if damage is already sitting on your units from a previous play — say a spell dealt 4 damage to your units earlier in the turn and they're still alive — playing Janna after the fact will heal that marked damage off them. Per Rule 418, healing clears marked damage from units.

Two exceptions where her heal can't help:

• Negative Might effects (like debuffs) are not damage — Janna won't remove them (FAQ #560).
• Elder Dragon's passive — even if you heal before the 1 damage ping, that 1 damage is considered lethal by his ability, so the unit still dies (FAQ #9964).