How am I supposed to go 4 seconds faster?! This seems impossible. What am I missing?

Little changes to driving make a lot of difference in the Boost Blitz challenges. Use the boost we give you liberally on the straightaways.  Keep an eye out for shortcuts -- on the higher levels you HAVE to take the shortcut to earn 3 stars.  Also, try using a character that has a speed up or boost ability (like Rez or El Zipo)