How do I have my Mac (that is hooked up to the web via ethernet) switch over to a 4G hotspot when the ethernet goes down? Automatically.
I suspect that you will need a specialized 4G/LTE failover router that senses the loss of the primary broadband that passes through it and internally fails over to the 4G/LTE connection.Cradlepoint, Peplink, Netgear, and others make these cellular failover routers.
The problem is that the Mac, from a hardware and networking perspective will not think the Ethernet is down, because it will most likely be talking to the home router and perfectly able to connect to other devices in your home. What I suspect you want to overcome is if the broadband service goes down on the other side of your home router.
In that case VikingOSX's suggestion is the best solution as the router is in the right position to know the connection to the internet has gone away.
However, if you want to cobble together your own solution, you might look at ControlPlane
<http://www.controlplaneapp.com>
This is a utility that can monitor various sources of information and determine which networking "Location" you should be using. One System Preferences -> Network -> Location can be configured for just the 4G hotspot, and another can be configured for just the Ethernet, including all authentication, proxies, DNS, etc... needed for that connection.
But figuring out what triggers will make the switch to and from the 4G hotspot is the tricky part.
Automatic 4G ethernet backup gateway
