Which bus I should take from Hong Kong Airport to 4 Cliff Road, Caritas Bianchi Lodge?

You can take the Airport Express Line from airport to Kowloon Station, and then walk to south around 300 m to take the bus line 203 E from International Commerce Centre Station to Kowloon Central Post Office Station. Then walk to north about 300 m you can see the hotel.

How about I want to take Airport bus to city which bus number I should take? A21?

You can directly take Airport Express to the city, which takes only 20 minutes.