A few questions about Beijing taxi

I would like to ask;

1. Can an airport taxi fit 4 persons if I have only 2 small luggage and 2 knapsacks?
2. Is the airport taxi fare standardized according to location it runs by meter?
3. If I take a taxi from the airport to the City Center at 5.30am, will I get caught in the rush hour?
4. Do taxis and buses accept cash?

Thank you.

Kiun
Malaysia