I assume you want $1, $2, $3 and $4 to be something other than "." and "-". If so, how about:

perl -le'
    $_ = "..."; 
    s/[\.-]+//g;
    if (/(\S)(\S)(\S)(\S)$/) {
        print "$1$2$3$4"
    }
'
[download]

or just

perl -le'
    $_ = "..."; 
    s/[\.-]+//g;
    print substr $_, -4;
'
[download]