Use the authpf_users table instead of adding rules for this.
in your main ruleset:
table persist.
pass in quick on $external_if from to 172.16.0.0/22
pass in quick on $external_if from to 172.16.4.0/22
pass in quick on $external_if from to 172.16.8.0/22
then pfctl -f /etc/pf.conf when you add a rule like that