Well, first off, their best magicians aren't likely to have 4+ power in anything. It's possible, but would be rather rare.
Anyway, though, I don't think that really works either. Magic isn't inherently divine or every mage would have priest power. The divine power is the thing the makes a Pretender God what he/she/it is. It infuses the priests with their abilities and provides the energy behind blessings.
When it comes to a Pretender's magic, there are really two measurements going on: one is the divine nature of the god (the power it can provide via blessings as well as the effects of its dominion) and the other is its ability to directly manipulate magical energy by casting spells. They happen to equal each other at first, which is logical enough, but can easily vary once the god's magical powers strengthen or weaken. This is the logic, as I see it, behind why changes in the magic levels during play do not change the potency of blessings. (Well, that and game-balancing!
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So, since having magical power is not inherently the same thing as being a Pretender God, I don't think independent mages should be able to provide blessing power. The "placebo effect" explains the +1 to morale, so I wouldn't say independents get any actual blessing at all, or at least not in the same way the followers of a Pretender God do.
An interesting addition, however, might be to allow the creation of a dispassionate or somehow otherwise limited God of the Independents. (It would have to be limited somehow, or else it would become simply another Pretender, and one that starts with possession of every neutral province to boot.) Possibly some faint vestige of the now "retired" Supreme Being's power could still linger, allowing those still faithful to Big Cheese (emeritus) some form of blessing? Designing that effect would add some player-generated variety to the independents, as well as satisfying both logical arguements and allowing people who prefer the current system to continue undisturbed.