Katt Williams better be saving his tour money because Uncle Sam is on his case. The comedian was just hit with a huge bill by the Internal Revenue Service.   Katt, real name Micah Sierra Williams, is accused of stiffing the government out of more than $4 million.  According to TMZ, Katt failed to pay $3.2 million in taxes in 2008 ... and another $829,352 in 2009. This is just the latest in a…