Suppose I have a force 4 astral armour spell sustained by a foci rating 4, with 4 sucesses

Its clear that if I go into an background count of 4 or more, both the spell and foci are toast.

If I go into a background count of 2 though, what happens?

In a BC of 2, the force of both the focus and spell are both reduced by 2. End result is you have a Force 2 Astral Armour spell sustained in a Rating 2 Sustaining Focus. As effective successes are limited by the force of the spell, it is effectively a Force 2 Astral Armour. As soon as you leave the BC, it returns to normal.