Using mask processor with a specific twist

[vicious1]

I have been struggling to write a mask processor command that accomplished the following:
I have a 12 character password, of which I know it contains the letters a b c d (for example)
How can I write a mp command that will generate 12 character passwords but all of them MUST contain those 4 characters at random places?

[hblender]

It is impossible to do it with the mask processor. Give a try to the wonderful reject rules https://hashcat.net/wiki/doku.php?id=rule_based_attack . Start with the rule named "Reject not contain" which rejects every password that does not contain those letters that you mentioned.