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 X^3  0  0  0 X^3  0  0 X^3 X^3  0  0 X^3 X^3 X^3 X^3  0  0  0  0 X^3
 0  0 X^3  0  0 X^3  0 X^3 X^3  0 X^3 X^3 X^3 X^3  0  0  0  0  0 X^3 X^3
 0  0  0 X^3  0 X^3 X^3 X^3  0  0 X^3  0  0 X^3 X^3 X^3  0  0 X^3 X^3  0
 0  0  0  0 X^3 X^3 X^3  0  0 X^3 X^3 X^3 X^3  0  0 X^3  0 X^3 X^3  0  0

generates a code of length 21 over Z2[X]/(X^4) who´s minimum homogenous weight is 18.

Homogenous weight enumerator: w(x)=1x^0+5x^18+10x^20+224x^21+10x^22+5x^24+1x^42

The gray image is a linear code over GF(2) with n=168, k=8 and d=72.
This code was found by Heurico 1.16 in -6.48e-008 seconds.