The tool used with OpenBSD for firewall operations is PF -- Packet Filter. However there have been many changes since your version of the OS, which is
unsupported.
There have been many, many changes to PF, so you will need older, out-of-date documentation.
I will build and attach a 4.3 version of the PF User's Guide for you, it will take a few minutes. Meanwhile...
The man pages for 4.3 might be on your system, or might not. Here are links to the two you will absolutely need for reference to PF, for 4.3:
pf.conf(5)
pfctl(8)