FE1 is the soft ride suspension
FE2 is stiffer than FE1. I believe it was used on CTS's that had the performance and handling package but didn't have the sport package.
FE3 is the sporty suspension.
FE4 is what was used on the CTSV
Theres also the auto leveling shocks for the rear but they aren't called FE5, at least I dont think. I'm almost positive there wasn't an FE5 package but I could be wrong in which case some one on here will correct me.
You can change to FE1 with no problem. Some people will suggest getting the correct springs due to the different spring rates between FE1-FE4. Others will say it's okay. It's really up to you.
