I just switched my RPi 4b to openrc-init.  I had previously moved my Gentoo desktop over to openrc-init following their instructions:  https://wiki.gentoo.org/wiki/OpenRC/openrc-init

The only thing that didn't work for me there was networking.  There I had to add my network interface to the default runlevel, then I was off to the races.

# rc-update add <my interface's start script in /etc/init.d> default

I pre-checked my RPi, and saw that network-manager was already in the default runlevel.  When I rebooted, it Just Worked(tm).

The only thing I did extra was to ensure that my serial console would work (in case networking didn't come up) by adapting the Gentoo instructions:

# ln -s /etc/init.d/agetty /etc/init.d/agetty.console && rc-update add agetty.console default

I literally just did all this, so if my RPi blows up later today I will post an update.