What is the difference between a Thule Apex for four bikes and a Thule Vertex for four bikes.
One is silver the other black and about $100 difference.

My guess would be the RDT (road dampening technology) is the difference.

https://www.etrailer.com/question-37652.html

The Apex 4 swing is awesome. Super easy to put on and remove. And once you compare the swinging to the side vs the tilting, the tilting seems ludicrous. Swinging gets the bikes completely out of the way for completely open access to the back of the vehicle. (Also, we have dogs that go in the back, and they can't jump over the tilting racks.)

More expensive though, But in my opinion totally worth the extra $120.

One note: Make sure it will actually fit four bikes. Our first four-bike carrier (don't remember the model) went back because we couldn't fit our four bikes -- modern bikes with wide tubing, weird shaped frames, different sizes, etc. This was even with all of the front wheels off. The second one was still a puzzle to get the bikes on, but they all fit.