The Domino geniuses here I guess will have some good advice on what to do, if anything with the underlying mail platform.
From a BES Perspective you'll want to backup your database at a minimum ... this
doc states its for Exchange and GroupWise, but some of the content is relevant for all platforms.
I'd also recommend printing out the 4.1.4 Domino upgrade guide to have with you as a checklist to go through. I print out an install guide every time I do an install to make sure I don't forget anything.