In mountainous and urban settings, a clear view of the sky can be an issue. While 3 satellites will give you a vaild GPS position solution, and 4 will give you a valid altitude solution, neither of these solutions will be ideal. This is a situation where more is better.
DJI quads will not go into GPS with less than 6 satellites in view.
Unless you plan on operating in a REALLY deep canyon or valley, you shouldn't have too much of problem getting lock on 6 satellites. It might take some extra time, but it will likely happen.
If you can't get a sufficient satellite lock and you absolutely have to fly there, you can get one of the higher quality flight controllers, such as the WooKong or A2, that have a better GPS receiver in them. They cost a lot more, but you will definitely see more satellites.