I was charged nearly 4 euros for a journey between the Cathedral and Plaza Nueva.
I didn't realise that it was so near.

The journey took only 1 minute and the fare was almost 4 euros.

Was I ripped off?

I suppose you got off the airport bus and then went to Plaza Nueva.

It's only about 250 metres so as long as you haven't got loads of luggage it is better to walk.

You weren't  ripped off because you were charged the minimum fare.

However maybe the taxi driver should have just not charged you.  

There is some info here about taxis and taxi apps in Granada

https://granadaforo.granadainfo.com/getting-around-in-granada/uber-in-granada/