I am adding 4 new circuits to my house. Do I need separate neutral and separate ground for each circuit, or can they be shared?

Depends on what you are talking about.

Foe example, if you are going to use four separate circuits, then yes, each circuit must have a netural and an equipment grounding conductor.

However, if you are going to use a multi wire circuit with a shared ground, that also is acceptable.

Several words of caution, if you choose to do a multiwire circuit then you need to use a double pole breaker that will trip and open both lines at the same time. Also, be aware, if you use a multiwire circuit and for any reason the neutral fails, you will drop 240 VAC across the circuit.

If at all possible, use separate neutrals for each circuit.

Shared neutrals in a multi-wire branch circuit can pose serious problems if the neutral is opened by a loose connection.