Router <-> Switch <- 4 pcs
simple enough get a mini switch 8 port connect the 4 pcs to it then connect the switch to the router like a computer would be normally, your router remains connected to what ever your ISP provides, now you can share the files between the 4 computers
using windows 10 set up a group on one computer remember password, then main other 3 pc connect to group make share folders public to group only and there you go....