This does it!

Thank you all, you are may today's champions!

Switch from the old 'unbound' mode (see image) to the new Python mode.

Thank you for trying to help. I'm feeling kinda dumb right now.

I select Firewall - pfBlockerNG. I now see a menu line with items General, IP, DNSBL, etc. There I select DNSBL and get a configuration screen. Neither in this nor in any of its three subscreens there is an item "AAAA". I even searched for the string.

Maybe there is a problem with my configuration? I let the wizard create a default configuration after I installed pfBlockerNG yesterday. After this in the services widget the entry pfb_filter is shown as running, whereas the entry pfb_dnsbl is not. I cannot start this service from the widget.

I must be missing something totally obvious.

Go into DNSBL and enable "no AAAA".
When you enable it you get a new section called Python no AAAA List.
Domains you put there will only resolve IPv4.

SO, is there a way to make pfsense only resolve ipv4 for certain domains?

and block AAAA request ?
pfSense, aka the resolver will do its job as asked.
You could probably do something with domain overrides or install pfBlockerng and use this option :

as it was included just for that : block AAAA requests of all domain names listed.

edit : Non, forget about host overrides.
You probably have to pick the correct unbound's config settings, see https://nlnetlabs.nl/documentation/unbound/unbound.conf/ )