It's also probably not worth it either unless they have very large pistons - alot of 4 pot calipers which could be used would be considered a downgrade in performance.

I've got a set of 4 pot AP Racing calipers off a McLaren and whilst I could get them to physically fit quite easily the pistons are too small to be of any performance upgrade. Any better caliper you're considering fitting to an E53 4.6is/4.8is would need to be at least a 6 pot or 8 pot based setup to improve performance.