Pihole is runs on pi zero’s I am sure you would not have any problemswith running it on your pi 4’s. I have my primary pihole instance running on a pi 5 with a bunch of other things, my secondary pihole instance is running on another pi 5 that used to run ghost which hosts my webpage.

I would say put pihole on both, also put unbound onto whatever one you decide is your main pihole. If you are going to get pihole and setup Matrix with the Whatsapp bridge may I suggest you get a firewall as well, something that you can flash OpenWRT onto. I have a Flint 2 that I have multiple vLANs setup on for security, but also a VPN so that I can have the benefit of all my services away from home. It is something that I wish I had setup a lot sooner.

Of course you will need a static IP address for this, for me my ISP gives them out for free but I understand that it is expensive elsewhere. I do however hear there are apps that emulate static IP’s for things like wireguard to use.