It depends on whether you want to give the officer a reason to ticket you or not. I'd probably go with all four, myself.
I decided, after the fact, to put none of the four stickers on my Salsa Baby. I can't remember the last time I was on a freeway during commute hours. It's just not worth the potential damage to the car's paint. Replacing an expired parking permit sticker brought home the point. Come January 1, 2008, I don't want to see four HUGE discoloration marks.
