BEST ROOT APP FOR NOTE 4 SPRINT?

I don't know the Sprint Note 4, but since you are on Android 6 an app is unlikely to be the way to root. They relied on security holes that were plugged in 6.0, so really only work on older versions.

Hi, I use CF_Autoroot it's really easy to use, you can find the download for your model here. Phil
http://download.chainfire.eu/585/CF-Root1/CF-Auto-Root-trltespr-trltespr-smn910p.zip?retrieve_file=1

Rooting your Note 4 is a good project for an older phone as you don't have to worry about warranty issues or Sprint sending OTA updates, but it's most definitely not a quick fix for your problems. Rooting gives you a lot more access to the internal working of your phone but once you do that puts all the responsibility on you for maintenance and upkeep. It's also gives one more ways to brick their phone if they're not careful.
As for wanting a quick and easy way to root your Note 4, there are one-click apps for this, but keep in mind those easy-to-use rooting apps also compromise your personal info by introducing background processes that data mine your phone. That's a big minus with the silver lining that once rooted you then have the ability to block or remove those issues.
I'd recommend a safer but not as simplistic process to root your Note like this one:
https://androidmtk.com/root-samsung-galaxy-note-4-sm-n910p-sprint

That said, looking up the specs for a Note 4 it came with 32GB of internal storage. You've either got a lot of media stuff (photos, music, video, etc.) that can be moved over to your microSD card, (without the need to root) or that's just a massive number of apps. What does your Settings >> Storage >> Internal storage menu reveal?
Samsung includes a basic file manager app that you can use to transfer files around. Look in your apps menu for the 'My Files' app. You might also want to install this 'DiskUsage' app. It will show you a graphical image of your phone's internal storage, sometimes it's easier to have a visual representation of how much stuff we all accumulate on our phones:
https://play.google.com/store/apps/details?id=com.google.android.diskusage&hl=en