:g/^/exec "s/^/".strpart(line(".")."    ", 0, 4)

This runs the exec command on every line that matches /^/ (matches every line).
The exec command takes a string and executes it as if it were typed in. 
line(".")."    "           number of the current line plus four spaces
strpart("123 ", 0, 4)      first four characters ("123 ")
"s/^/123 "                 substitute the beginning of the line with "123 "