for stake pool registration you have to provide a public IP address of you relay node.
pool-relay-ipv4 - relay node ip address
I have a public IP - but that is a dynamic IP - it is changed by the ISP every week. Is it possible to use domain instead of IP address?
Im pretty sure that you need a static one, maybe you could try to use ddns.
dynamic DNS? yes I would use it, but that would mean a domain name could be added during registration - the question is - is it possible?
Im not really sure. I guess you have to wait for someone with a bigger technical knowledge than me 
You can register your pools relay on chain as hostname and portnumber. So what you need in case your IP changes is a quick way to dynamically update the DNS A-record for this hostname to the new IP.
The next challenge is to also end up in other relay node’s peer list.
First the background: the implemented block and TX propagation protocol is made in a way that the receiver ask the sender for certain data. It’s not the sender who can blow out as much as he can, demanding ressources for all receivers to validate the incoming data.
Said this I haven’t tried it yet, but I would try to publish a hostname to other peers, with a relative low TTL, eg 2-5 minutes.
What needs to be tested is if the implemented cardano-node does consider this relative low DNS based TTL, and do a fresh DNS A IP lookup
ok - so although the param name is pool-relay-ipv4 can I give host name as a value instead of a IPv4 address?
Some IPSs will provide a static IP for a fee.