In this case, the fourth day of Lunar New Year is automatically counted as a statutory holiday in Hong Kong.
If you have been employed under a continual contract for at least three months in Hong Kong, then you are entitled to the holiday pay.
Yes. Private companies in Hong Kong do have the right to decide their own schedules and allow additional holidays to their employees on top of the official public holidays set by the government.
If a designated public holiday falls on a Sunday in Hong Kong, the immediate following Monday would be a public holiday according to Hong Kong laws.