SAP智力題-----A specail mouse

There are 12 mice, only one of them has a different speed to eat cake. How many cakes do you need to find out the special mouse? How about 5 cakes? What if only 4 cakes?
(Note: when you let mouse A eat one cake, B eat another, if A finishes firstly, then A is faster than B.)
https://www.sdn.sap.com/irj/scn/forum?forumID=410