Mattis Hasn't Decided to Send 4K More Troops to Afghanistan

Mattis is essentially waiting for Congressional approval at this point. The budget hasn't been approved. The conversation between him and Tulsi Gabbard during the last round of Congressional hearings gave an indication that the funding is not guaranteed as it has been in the past. Mattis basically had to admit that the defeat of Al Qaeda is not, nor has it ever been, in the past 16 years, the priority of the DoD or the primary use of the budget allocated specifically for that purpose.
I think he also knows that 4k troops is woefully inadequate and only serves to provide a focal point for non-state actors with a grievance against the US. Meanwhile, Al Qaeda and ISIS continue to operate with our tacit support in Iraq, Syria, and Yemen...