Search Multiple DB, NOT Multisite

Hello everyone, hope you are all doing well.
I have Googled this and went through the forums the best I could but it seems I cannot find anything that is quite like I am asking for.

I have a client that has 4 sites, different tld’s but all on the same server.

He would like it so, when a search is done on one site, results will show from that given site plus the other sites. All sites are on the same server but it is not a multisite.

Does anybody have an idea on how one could go about this?

Thank you