Why is horizontal stabilizer set to 4° up in airbus aircraft for take off?
Both landing and take-off are critical phases of flight, and it is standard procedure to configure the aircraft for those phases to reduce the workload on the crew.

On landing, aircraft are configured for a stabilized approach, which means that the aircraft can be flown to the runway with a minimum of configuration changes to flaps/slats, trim, power, and drag devices.

On take-off, the aircraft is configured for take-off, and trim is set so as to provide a slight nose-up. If the trim is set nose low, it requires a lot of pull to "unstick" the aircraft from the runway. If the trim is set too nose high, then the aircraft will want to nose up before the aircraft may be ready to fly, and then to continue a nose-up attitude after rotation, requiring more than a minimal effort to keep the a proper departure attitude. 

Nose-up trim can induce what is called a trim stall, which is when the trim is set so far back that the aircraft will exceed the critical angle on take-off:


  ELEVATOR TRIM STALL
  The elevator trim stall maneuver shows what can happen
  when full power is applied for a go-around and
  positive control of the airplane is not maintained.
  [Figure 4-8] Such a situation may occur during a
  go-around procedure from a normal landing approach
  or a simulated forced landing approach, or
  immediately after a takeoff. The objective of the
  demonstration is to show the importance of making
  smooth power applications, overcoming strong trim
  forces and maintaining positive control of the airplane
  to hold safe flight attitudes, and using proper and
  timely trim techniques.
  
  Source: Airplane Flying Handbook p. 4-10