LA to Mexican Riviera - boarding time?

If ship leaves San Pedro at 4:00, when do I have to be on board?

At least an hour before sailing. At that time they have to send the sailing 'list' to the government. I would be on by lunch and enjoying the ship.

In San Pedro you can arrive anytime from around 10AM until one hour before boarding.

For your cruise that would be 10 to 3.

If you don't have any plans before you board, get in line before 11 and head to the Main Dining Room (usually the one on deck 5 - IIRC) and have a nice relaxing sit down, waiter served lunch.

1 hr before it sails.

If ship leaves San Pedro at 4:00, when do I have to be on board?

Like others have said no later than 3pm. Muster will most likely be at 3:15, with a follow up muster for late boarding passengers. You can arrive as early as 10am and boarding normally begins around 11:30am.