>perl -wMstrict -le
"for (@ARGV) {
   (my $sp = $_) =~ s{ (.{4}) (?= .) }{$1 }xmsg;
   print qq{'$_' -> '$sp'};
   }
"
""  A  ABC  ABCD ABCDa  ABCDabc  ABCDabcd  ABCDabcdE
'' -> ''
'A' -> 'A'
'ABC' -> 'ABC'
'ABCD' -> 'ABCD'
'ABCDa' -> 'ABCD a'
'ABCDabc' -> 'ABCD abc'
'ABCDabcd' -> 'ABCD abcd'
'ABCDabcdE' -> 'ABCD abcd E'