that was also my first idea: which are the elements in the matrix to multiplicate? the last row? …but
i found a sgi example. in the example they use the feedback mode but i didn’t find tutorials on it - only the example is hard to understand.

float xp = mv[0] * x + mv[4] * y + mv[8] * z + mv[12];
float yp = mv[1] * x + mv[5] * y + mv[9] * z + mv[13];
float zp = mv[2] * x + mv[6] * y + mv[10] * z + mv[14];
float wp = mv[3] * x + mv[7] * y + mv[11] * z + mv[15];