Can you board the ship before this 4pm?
You have to be checked in no later than 60 minutes prior to scheduled departure time. If not you may be denied boarding. Look here
for more details. Even if it says it starts at 1:30pm, it is common to start much earlier than that. The cabins may not be ready until 1:30pm but you can still board and eat lunch and look around, have a cocktail, etc.