>perl -wMstrict -le
"my $text =
   qq{XXXXaXXXXbYYYYYcYYYYYdXXXXeXXXXfgXXXX\nXXXXhXXXXiYYYYY};
 print qq{[[$text]]};
 ;;
 my $result = join '', $text =~ m{
     (?<= (?<! [[:upper:]]) [[:upper:]]{4})
     [[:lower:]]
     (?= [[:upper:]]{4} (?! [[:upper:]]))
   }xmsg;
 print qq{'$result'};
"
[[XXXXaXXXXbYYYYYcYYYYYdXXXXeXXXXfgXXXX
XXXXhXXXXiYYYYY]]
'aeh'