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Substitutions at character positions are often easier with sed
sed '/^001/ s/\(.\{4\}\)../\1 /' file
The \1 puts back what matched within the \( \) and that is the first 4 characters.

Try this (untested, try with nawk and /usr/xpg4/bin/awk)
awk 'BEGIN{OFS=FS=""} (substr($0,1,3)=="001") {$5=$6=" "} {print}' a.txt

You say 001 but in your code you have 123?