The use of column-major format in the spec and blue book
X_x Y_x Z_x T_x
X_y Y_y Z_y T_y
X_z Y_z Z_z T_z
W_a W_b W_c W_w

0 4 8 c
1 5 9 d
2 6 a e
3 7 b f

float vec4_length3(float v[4]);

float M[16] = {...};

float T_length = vec4_length3(&M[c]);