The four countries — Niger, Mauritania, Mali, and Libya — are all problematic and potentially unstable. Their governments aren't necessarily in immediate danger of falling to Jihadi organizations. But they're the types of places where Islamist groups are strong enough, and the government is weak enough, to potentially send the country on the path to state collapse or civil war, barring the kind of high-level security assistance the U.S. is capable of providing.
Niger is home to some of the world's largest uranium reserves, thanks to the facility at Arlit — which was the target of an Al Qaeda affiliate attack in May of 2013. Later that year, 22 terror suspects were freed during a prison break in the capital of Niamy, an operation that an Al Qaeda splinter group orchestrated.
Mauritania is a restive multi-ethnic state held together by a deeply corrupt yet relatively pro-western military regime. But Mauritania is a safe haven for Al Qaeda in the Islamic Maghreb, a development that apparently overrides the moral hazards of training its military, at least in the minds of U.S. planners.