Is my phone using LTE if the notification area says 4G?
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.