I want to root Galaxy Note 4 so I can use a freezer app to keep rarely used apps running in background even after I stopped them. I don't have a clue have to do it. Please help.
You don't need root to do that, you can install a disabler app which does the same thing. Don't know what a "freezer app" does. As others have destroyed their phones after rooting because they didn't know what they were doing, I would suggest don't.
I want to root Galaxy Note 4 so I can use a freezer app to keep rarely used apps running in background even after I stopped them. I don't have a clue have to do it. Please help.
Without knowing just which model of Note 4 you have it's hard to be more specific but I'd recommend you go to the XDA Developers site and just do a search using your model. An example being: https://forum.xda-developers.com/note-4/help/sm-n910-cf-auto-root-t2897428
(Both XDA Developers and Chainfire (the suggested utility in that page) have a long history as reliable Android development advocates.)
Assuming you intend on installing Titanium Backup with your reference to 'freezing', I think that's a really good option. Not only does TB allow you multiple layers of backups, the 'Freeze' and 'Defrost' options are a safe way to experiment with will and will not disable your phone as whatever you do (well, within reason) can be reversed.
I wouldn't risk rooting your device if you're not familiar with the process. Just go to the Play Store and get Package Disabler Pro - does exactly what you need and is kept up to date thanks to great developer support.