>perl -wMstrict -le
"for my $text (qw(
   XXXXaXXXX XXXXaXXXXxyXXXXbXXXXxZZZxZZZxYYYYY
   XXXXxZZZ ZZZxXXXX XXXXxYYYYY YYYYYxXXXX
   XXXXxyXXXX XXXXxyXXXXxyXXXX
   YYYYYaYYYYY ZZZaZZZ)
   ) {
   my $result = join '', $text =~ m{
       (?<= (?<! [[:upper:]]) [[:upper:]]{4})
       [[:lower:]]
       (?= [[:upper:]]{4} (?! [[:upper:]]))
     }xmsg;
   print qq{'$text' -> '$result'};
   }
"
'XXXXaXXXX' -> 'a'
'XXXXaXXXXxyXXXXbXXXXxZZZxZZZxYYYYY' -> 'ab'
'XXXXxZZZ' -> ''
'ZZZxXXXX' -> ''
'XXXXxYYYYY' -> ''
'YYYYYxXXXX' -> ''
'XXXXxyXXXX' -> ''
'XXXXxyXXXXxyXXXX' -> ''
'YYYYYaYYYYY' -> ''
'ZZZaZZZ' -> ''