Is my phone using LTE if the notification area says 4G? When I bought my Galaxy S5, it came with Android 4.4.2. When I was connected to my mobile data network, the notification area had a little icon that said "LTE" above to arrows showing data being uploaded and downloaded.

Recently, I rooted and upgraded my phone to Android 5.0. Now, that same icon in the notification always shows "4G" above the two arrows when I'm on the mobile network.

I looked up the difference between LTE and 4G, and if I understand it, LTE is a subset of 4G, but a faster and more advanced version. Please correct me if I am not getting this right.

Assuming I'm close enough to the truth, one possibility is that I'm using the same connection, and it's just the icon that has chanced. Another possibility is that something about my upgrade has changed how my phone communicates with the network, and maybe I'm not getting the same connection.

In either case, how can I be sure I'm getting the best connection that is offered by my mobile provider?
LTE actually does not meet the original 4G standards, it is neither faster nor more advanced.

I'm pretty sure that it's just an icon change, but if it's not it's almost certainly that your connection is better rather than worse.

I recently saw this on my device as well and used SpeedTest to check how the connection was performing.  My download and especially upload speeds were absurdly high.  You could use it to see whether your speeds align with your expectations as well.