The generator matrix

 1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1
 0  2  0  2 2a 2a 2a^2+2a+2 2a^2+2a+2 2a^2+2 2a^2+2  0  2 2a 2a^2 2a^2+2a+2 2a^2  0 2a^2 2a^2+2  2 2a
 0  0  2 2a^2+2 2a^2 2a^2+2a  2 2a^2 2a^2+2 2a^2+2a 2a 2a^2+2a 2a 2a 2a+2 2a+2 2a+2 2a^2+2 2a 2a+2  0

generates a code of length 21 over GR(64,4) who´s minimum homogenous weight is 136.

Homogenous weight enumerator: w(x)=1x^0+105x^136+266x^144+3584x^147+42x^152+21x^160+77x^168

The gray image is a code over GF(8) with n=168, k=4 and d=136.
This code was found by Heurico 1.16 in 0.00402 seconds.