I just changed my front lines for stainless a couple weeks ago. As I was riding home tonight I was thinking how amazing my front brake feels. It's the best thing I've done for the bike, better than the exhaust system.
Changing the lines is no big deal as long as you, 1. are cautious with the brake fluid (it'll ruin paint, and it's not too nice to plastic either), 2. do a thorough job of bleeding the system once the lines are installed.
I can't comment on the 4 year replacement interval for the stock rubber lines. I can, however, heartily recommend an upgrade to stainless. At the very least, you should be changing your brake fluid completely every couple of years.