$ perl -le '$_="1 2 3 4 5 6"; s/^((?:[^\s]*\s){3}[^\s]*)\s/$1\n/ and print'
1 2 3 4
5 6

##</code><code>##

s/^([^\s]*(?:\s[^\s]*){3})\s/$1\n/

##</code><code>##

s/
    ^             # Start at the beginning.
     (            # Start capturing.
       [^\s]*     # 0 or more non-space.
       \s [^\s]*  # A space and 0 or more non-spaces... once.
       \s [^\s]*  # twice.
       \s [^\s]*  # three times.
     )            # Stop capturing.
    \s            # Match another space.
/$1\n/x           # Replace with what we captured and a newline.