What we want to do now is switch to 4 data and 1 log LUN for every DB
Why do you want to do this to all databases? I guess managing will pose a problem later with so many data files. I would identify top 5 or 10 percent databases which are heavily used and would split those databases into multiple filegroups.
Also what i understand is you are going to increase 1 datafile to 2 and 1 log file to 2 each. Adding another log file wont help you at all in performance as writes to the log files are sequential. Until one of the log files get full, sql server wont write to the other file. so you wont achieve any performance improvement here.
Also, i suppose you are planning to create seperate filegroups onto the new LUNs that u're planning to add. This way you can separate tables on different luns OR you can have tables on one LUN and indexes on other. This way you can achive good performance due to IO bottleneck.
My understanding is that SQL only grows 1 file at a time, thus proportional fill is lost as soon as all 4 files fill up and SQL only grows the one.
This happens with log files and not datafile. say your database has 2 datafiles with 100mb and 200mb in sizes. If you're entering data worth 6mb in size, according to proportional fill algorithm, 2 mb of data will go to the first file and 4mb to the 2nd file (1:2).