In almost all cases, however, the mark wouldn't be 4 times as effective. A normal paladin's challenge can only trigger once a round. A fighter can only use CC once a round. A warden could use his interrupt or reaction only once a round.
Of course the plain old mark effect does apply several times. However, it imposes a -2 penalty to separate attacks, rendering each less effective. Similarly, if you could grant resistance to all damage, that resistance would apply to each attack separately. That's actually very similar to what hospitaler's blessing does; except the mechanics are subtly different: instead of granting resistance to the attack, it heals damage per attack.
Now, it's perhaps not a wise mechanic, and perhaps Hospitalers blessing should be fixed again - but as is, it clearly applies to each attack and will apply several times when there are several attacks - just like resistance does.
If there should be a change; it should be to the Hospitaler, not the general notion of an attack or of a mark. Granting resistance is strong, and healing can even reduce damage below 0, so that's even better. On the other hand, let's not exaggerate the effect here; the monster can easily avoid this healing by not violating the mark - that's why these mark penalties are so high in the first place. Further, the hospitaler isn't the only marking defender that has the ability to severely cripple foes that violate the mark.
Getting the blessing right is a matter of balance - I don't think tweaking the interpretation of marks and attacks is the best way to address that balance.
Edit: To be clear, I (now) realize you're suggesting a house rule, but I just don't think the Hospitaler should be the inspiring example here. It's so far from ordinary that changing the way marks on monsters work in general because of the hospitaler is just approaching that problem from the wrong side.