Authorities are looking for four inmates who escaped early yesterday morning from a minimum-security federal prison in Sheridan.  Officials say the inmates, each of whom was incarcerated on drug convictions, were found missing around 3:30 a.m. yesterday during a head count.  The Federal Prison Camp is a work camp located next to the minimum-security Federal Correctional Institution in Sheridan.  The inmates are identified as Abel Mendoza, 34,  Rick Mendoza, 37,  Isidro Rivera, 35,  and Jose Cisneros, 42.  Authorities say none of the men has a history of violence.