Try this:
%RANDOM := 1D6
IF %RANDOM = 5 OR %RANDOM = 3 OR %RANDOM = 2 OR %RANDOM = 1 OR %RANDOM = 6 THEN $DEST := @P15
You have to continually reiterate the "%RANDOM" or the system won't know what to compare. Also, in this case, you're technically testing for any value other than four, so you can just replace that whole IF part with "IF %RANDOM != 4".