perl -F, -lape'join(",",@F[0..3],$F[3],$F[4],@F[4..$#F])' data.txt

##</code><code>##

perl -F, -MData::Dumper -lape'Dumper \@F' data.txt

##</code><code>##

perl -F, -lapi.bak -e'$_=join(",",@F[0..3],$F[3],$F[4],@F[4..$#F])' data.txt