Bounding CKM mixing with a fourth family

CKM mixing between third-family quarks and a possible fourth family is constrained by global fits to the precision electroweak data. The dominant constraint is from nondecoupling oblique corrections rather than the vertex correction to Z -> bb used in previous analyses. The possibility of large mixing suggested by some recent analyses of flavor-changing neutral-current processes is excluded, but 3-4 mixing of the same order as the Cabbibo mixing of the first two families is allowed.