Difference between Thread pool and asynchronous tcp server

Question

Hello,

I am doing some literature study on a new project that I will be starting soon.

I want to make a server application capable of handling a maximum of 4 clients. I want to be able to read and write to a client at the same time (don't know if this is really possible).

I read that using a multi threaded application is better, but I don't know if for example an asynchronous server is also multi threaded (I can't see any threads being created).

So my question is: What is the exact difference between a thread pool server and an asynchronous server?

The problem i a lot of questions araise while reading that I can't awnser myself.

Thanks in advance and with best regards.