The part about modems only permitting 4 devices is completely inaccurate. DSL and cable modems have one network jack— this connects to a router which has as many network jacks as you need. Most have four, more expensive ones have 8. If you need even more, you plug the router into a switch, which can have dozens.
Your ISP can't regulate how many devices you connect because it doesn't know how many devices are connected, your router uses NAT to make them all appear as one. All they care about is that you stay under your bandwidth limit.