Your configuration makes no sense and goes completely against the basic idea of Unbound. The purpose of Unbound is not to use a public DNS provider like Cloudflare or Quad9. The actual purpose is to query the root DNS server directly. Therefore, there is no forward-zone configuration, as Unbound automatically uses the root DNS servers.
Here is a corresponding list of root DNS servers. You will also find the IP 198.41.0.4 there. Root Servers
If you really only want to use Cloudflare or Quad9, then you don’t need Unbound and can map this directly via PiHole.
The configuration of PiHole as upstream DNS in Unbound is also questionable. Normally the sequence should look like this
Client > PiHole > Unbound > Upstream DNS