Covering array records: strength-4 covering arrays improving the best known sizes

Explicit covering arrays CA(633; 4, 100, 3), CA(644; 4, 105, 3), CA(908; 4, 7, 5), CA(1838; 4, 7, 6), CA(1966; 4, 8, 6), CA(3942; 4, 12, 6), CA(6453; 4, 26, 6) and CA(6581; 4, 27, 6), each improving the best known size in the ca-tools covering array table, obtained by post-optimisation of the published record arrays with C search programs written by Claude Opus, and verified exhaustively.