A much simpler and from VZW's perspective more important issue is that a user is far more likely to be watching Netflix or otherwise streaming large amounts of data on a 10" tablet screen than a 4" cell phone. Thus, tethering is likely to increase the demand on the network. The tethering charge is meant to compensate for that increased demand.
From a user's point of view it comes down to whether one's data consumption significantly increases from one month to another. Monitoring that activity is non-intrusive and if a user typically uses half a gigabyte per month and suddenly begins to use four or five gigs per month it's very likely that they are using the phone's data plan to display content on another device.
This issue came up a couple of years ago when unlimited data plans were much more prevalent than they are now. And I suspect that those of us who have managed to maintain grandfathered unlimited data plans may well be targets of special interest by carriers on this score. After all, if one opts for a tethering plan combined with a tiered data plan VZW provides an additional 2 gig ceiling on the plan. On the other hand, if one has an unlimited data plan unauthorized tethering amounts to "abuse" of that plan (in VZW's view.)
Curmudgeon Member of Rescue Squad
By "4G Trick", are you referring to the past practice where you ordered the WiFi Hotspot service, got unlimited data along with it, then cancelled the WiFi part of it, and ended with unlimited data?
If so, I thought that offer was over now.
Cleanse the gene pool,
Stamp out warning labels.