Or,
Attack roll exceeds AC resulting in a hit
Start hit resolution
Shield Interrupt resolution
AC = AC + 4
End Shield resolution
Attack roll no longer exceeds AC, void hit resolution
Interrupt goes off before the hit finishes resolving without pesky, "we went back in time" narrative.
Again interrupts by the wording in the compendium occur after the triggering event has started and before it finishes resolving. They don't turn back the clock.
Of course immediate interrupts turn back the clock. They turn back the clock for the players at the table. The DM has to re-adjudicate the situation because the situation has changed. If the hit no longer hits, then it IS turning back the clock. Not from the perspective of the PC, but from the perspective of the players.
Void to hit resolution. How is this calculated? The equation is used again to calculate to hit. Previous to hit plus previous pluses is compared to previous AC plus the II effect +4 bonus to AC.
Void drop to zero resolution. How is this calculated? The equation is used again to calculate damage. Previous damage done from previous attack is subtracted from previous hit points plus the II effect additional hit points.
You cannot change how you adjudicate the II from one to the other and state that you are correct.
Either you re-calculate how the trigger was determined in every case, or you are not being consistent and you are changing one example from an II to an IR.
Sorry dude, but your method cannot be called an immediate interrupt if you do the healing after the trigger is already set in stone. You have to re-calculate if the equation(s) to get to the trigger based on the effect of the interrupt and see if the trigger is still applicable or not.
"If an interrupt invalidates a triggering action, that action is lost." What does this mean? It means that the to hit roll still has to hit. It means that the target still has to be in range. It means that the attacker still has to be capable of making the attack. An action is an action. Every part of the action has to still be applicable for the action to be resolved. The interrupt invalidates the entire action if it invalidates any part of the action (to hit, damage, range, valid target), it does not invalidate just a small piece of the action.
If any of this changes because of the interrupt, it interrupts the attack. That's why the "+1 to hit bloodied foe" example negates the attack. The foe no longer gets the +1 to hit because his foe is no longer bloodied. The scenario for which the action was being used is no longer the same.
Ask yourself the following question. What happens with the following immediate interrupts?
Trigger: You are hit by an attack
Effect: The attacking foe is stunned until the end of your next turn.
Trigger: You drop to 0 hit points or fewer
Effect: The attacking foe is stunned until the end of your next turn.
Why would the first one completely negate the attack and the second one not completely negate the attack? Give the explicit rules quote to support your answer, don't just make it up.
The trigger "You drop to 0 hit points or fewer" is not set in stone. It's determined by the result of the immediate interrupt, just like "You are hit by an attack" is not set in stone.