When you say 'identification of human beings', what type of image detail do you desire? 1, 2, 3, 4:
To give you a rough sense of what you are looking at / for. At 4000m, with a 1080p camera, and a huge 1000mm lens, best case scenario you will get this:
I am pretty sure it will be worse than this because of lensing / atmospheric issues at such super long ranges plus really long lenses tend to have high f stops and/or are fantastically expensive.
For more color, take a look at: View A Face 3000 Meters Away.
In general, you should be looking at long range military focused products. For example, Vumii.
You probably have to prove it's human being crosing the border, and get a Johnson rules footprint not to identify them. As John says it's a differnent story especially in darkness
What soldier use in Afghanistan, to identify Allies or ennemies and if your carry weapons (and sometimes shoot friends, oups..) are thermal cooled systems with very long focal. See on Flir side (35 to 60 K$ per camera)
Not sure you will be able to distinguish Mexicaners from US citizen if they don't wear sombreros..:-)
I heard back from Michael Mojaver of Apex Corporation, who told me that Apex has a customer that OEMs some technology from Apex, and produces cameras and surveillance platforms designed for border surveillance applications as well as other long range applications.
VIT Security Group is the company and Longview 2 is the product.