If I want to host a hidden service but I don't want people to know that the server is communicating to Tor, what can I do? Can I use obfs4? How to setup things?
Yes, bridge and pluggable transport users are able to provide onion services over them.
Bridge users are just taking some extra steps to pick their guard (the bridge), use their guard as their directory server and possibly to use a pluggable transport to transform the traffic while it traverses the network to the guard. Outside of that there is nothing fundamentally different with how Tor operates that would interfere with running an onion service.
However, pluggable transports are built to defeat DPI. That is, upon naive inspection by some algorithm it would not look like a Tor connection. It would likely not stand up to an in-depth analysis of the traffic. It would not be impossible for an observer to determine that it was communicating with the Tor network.