Just to be clear for others reading this thread, (I think this is what you have set up), FileMaker Pro can host a database on one computer if you use FIleMaker to open the file on that computer and enable sharing both for the application and the file. Do NOT, try to use filesharing and have users access a shared directory directly to access and open the file as this can damage the file.
What you describe should work just fine up to a total of 4 other devices connecting to the 5th that serves as the host machine. You "lose" a number of server features--such as automatic back ups, scheduled scripts, Perform script on Server, user statistics and better protection from damage in the event that a client experiences a crash or force quit of Filemaker.
But you also save major $$.
The most important thing is to set up some kind of back up system where a script uses Save A Copy as to save a copy of the file at least once a day. These files should not be kept on the same computer as the hosted file. Either save across the network to another folder or set up a back up utility to back up this back up file to a remote location.
Here's a thread that shows a script for saving a copy of the file every 15 minutes. That's for a file undergoing continual development--not something you need here, but you should be able to adapt elements of it for what you need--especially how each back up copy get's a unique file name: Saving Sequential Back Ups During Development