Anyone know, how to do coregistering using four images.
That I want to know is like the metric alignment for four images,
You could pick one image and register the other 3 images to it.
One way to make the choice less arbitrary could be to compute all 4 choose 2 pairwise registrations and choose whichever of the images has smallest mean deformation needed to align it to the other three images.
The other approach is atlas construction, but I guess four images is too few to do that.