We're actually going from 1 data file to 4 data files per DB. The log file is staying the same at a single file per DB. This will give us 5 files per DB, each one on a separate LUN.

The goal isn't SQL DB performance improvement, it's actually SAN controller performance improvement by spreading the IO for the DBs over multiple LUNs instead of just the 2 we currently have (1 for log, 1 for data). TBH, I'm not convinced that this is going to help us, but the decision is over my head, I'm just doing the work, and looking for the best way to do it.

So to have the DB grow all 4 data files uniformly we need to start out with files that are the same size. Are you saying the SQL will grow all data files at the same time when the DB is full (I'm assuming autogrowth is enabled here).