I'm waiting for my Mavic to arrive and I have an "old" version of DJI GO installed on my phone. I used that app for a looong time with my P3S and never updated it because it works great and reliably (and I have had bad experiences with updates).
I've read that GO 4 has a lot of bugs.. Should I use my old & reliable app or just install the new one? Can I keep both and just try which one works best for me?

Thanks!

Use the DJI GO 4 App, there are not a lot of issues, you will just see more post about it because everyone else flying on the go4 app is out flying their Mavic. DJI GO4 is the most up to date and best support for your Mavic. I have not had 1 Problem with it.

I don't believe there's a choice any more; I think DJI Go is no longer supported with the Mavic but only (if I recall correctly) for some of the older Phantom models.