How to make a server for more than 4 people?

With a server-side mod, you can have up to 10 player slots. However, only five will be able to play - this is hard-coded limit in the game.

There is a mod which allows for up to 8 players, but it requires both the server and all the clients to use custom .dll's (you cannot join normal servers using these dlls, so make sure to backup the default ones!). Of course, since it relies on replacing dlls, future alien swarm updates may break it.

Without modding anything -- the default campaign only allows for 4 players. Custom campaigns, however, have the option to allow up to 5 people in a single mission.