For those still stuck in 5.8-dom, it's possible to do this substitution without use of the  \K special escape of 5.10:

>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'
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This approach is likely less efficient due to use of a capture. Is it possible to do this operation in a pre-5.10 solo substitution (i.e., no  m//  unpack  join etc.) without a capture and in a fairly simple way (for some definition of 'fairly simple')?