As far as I'm aware, there is no official hierarchy for the order of out-of-turn action types in 4th Edition. But maybe we can figure one out, given the contextual clues available.
The full list of out-of-turn action types:
- No Action
- Free Action
- Immediate Interrupt
- Immediate Reaction
No Action is just that--an automatic effect that occurs due to the trigger, which is often used just to avoid a scenario where the triggered power can't go off due to stunning. This is commonly seen in monsters like the Balor, which explodes when it drops to 0 hp (MV, p.49). I could also see this working for objects or other elements of the environment that aren't alive or conscious, and so wouldn't be assigned actions at all. For example, if a PC smashes a steam pipe with a sword, steam would start spraying out afterwards, resolved as a No Action.
Free Actions aren't quite as instant.
Free actions take almost no time or effort. (Insider Compendium, "Free Actions")
The key word here is "almost," meaning that they do take a small amount of time and/or effort, whereas a Balor could be fully helpless and still have its Death Burst power go off.
Immediate Interrupts actually happen during their trigger.
An immediate interrupt jumps in when its trigger occurs, taking place before the trigger finishes. If an interrupt invalidates a triggering action, the triggering action is lost. (Insider Compendium, "Immediate Actions")
and Immediate Reactions take place afterward. Since you only get one immediate action per round, the argument seems pretty plain that immediate actions take measurably more time and effort than free actions, so it'd make sense they'd be slower (except for interrupts, which explicitly state that they occur during the trigger, as mentioned above).
An immediate reaction lets a creature act in response to a trigger. The triggering action or event occurs and is completely resolved before the reaction takes place. (Insider Compendium, "Immediate Actions")
So the order of operations here seems to be as follows:
- Triggering event - A PC attacks a monster.
- Immediate Interrupt - one of the monster's allies might step in to try to block the incoming blow. For the sake of my example, let's say it fails. Afterwards, the triggering event in step 1 finishes, damage is applied, and the monster becomes bloodied.
- No Action - Expending no effort to do so, the monster reacts to being bloodied by, say, spraying its acidic blood all over the triggering PC.
- Free Action - if he survives, the PC can follow-up on the initial attack with something like Press the Advantage. It doesn't take MUCH effort or time, but it still takes SOME, which would place it squarely behind a No Action.
- Immediate Reaction - One of the PC's allies can react to the initial triggering event with something like the Warlord's level-7 power, Join the Crowd.
This is all conjecture, but taken from how the individual actions are described. If there's still disagreement at your table (or if two out-of-turn actions are in conflict, and they're both the same action type), you might compromise by having the creatures involved roll a quick initiative contest to see who acts first. Or hey, they might go off simultaneously, if appropriate. DM's call.