The US military does not have the numbers to win a decisive victory in Afghanistan.
That depends on tactics. There's a certain Roman phrase "create a desert and call it peace".
I'll agree though that we won't win a decisive victory using current tactics. But that's OK. We can just keep on killing people forever.
Four thousand more troops won't cut it.
We do have a force in place to kill any suspected terrorists. However the majority of our troops there, including all of the ones that are being added, are there not to fight but rather to train up the Afghan military so they can fight for themselves.
A diplomatic resolution with one faction of the war, the Taliban, isn't likely because they know they can outlast the US at the level that it is waging the war, so they refuse the US' overtures.
The Taliban is wrong. We can last forever at current combat levels. Especially dronestikes. We can keep dronestriking suspected terrorists forever.
Against Al-Qaeda and ISIS, and killing terrorists, all the US can do is fight it's low intensity war that's bleeding a dizzying amount of money, and a body count, both American military personnel and civilian, which the US public is willing to tolerate.
I don't have current financial costs at hand, but I think we can afford it.
Very few Americans are being killed or maimed by the current level of conflict. We can maintain the current combat intensity forever. Dronestrikes in particular usually result in no American dead at all.
If you refer to non-American dead, I don't mean to sound cruel, but they did start this war by coming to our country and massacring thousands of civilians. The number of non-American dead is a matter of no significance, even if the number is very large.
Of course I don't mean to count our friends and allies in that. We want to minimize the carnage suffered by our friends and allies. It is the dead among those who oppose us who don't matter.