Normally a ship arrives and departs from a port on the same day, it is rare to have an overnight turn-round. Often when you see an arrival time that early it is not the time at berth, it is the time the ship embarks the port pilot- in this case, off Cascais in the Tagus river. It is then about 1 hour of pilotage onto the berth at Lisbon- just checked my voyage logs.
The usual rule is then to allow one hour after the ship berths because there is all manner of paperwork which the ship has to complete with a port, and the gangways to be put in place, and luggage begin to be transferred off the ship.
But all of the pilotage and paperwork will have happened the previous morning.
There have to be security staff there to open the terminal.
However I assume you are on the Oceania Marina disembarking on 23 May. She is, I see, due to have arrived at 10am the previous day at berth TPSAJ, which is Terminal 4, opposite Santa Apolonia Railway Station.
Having been there overnight the gangway will have been open all night, so staffing will be in place, so all of that is OK.
I would check with the ship, but you may have to carry your own luggage off, leaving the ship so early.
It is a small terminal building, so it is a short walk along the quay to the back door of the terminal, straight through and out onto the street.
There is a public bus stop straight outside the front door of the terminal, but they won't be operating at 4.15am.
Normally there would be lots of taxis there, but with neither the main line or the metro station open so early, it probably is best to either book a shuttle or an Uber.