Dear All,  
 
if I have an array of 4 Parabolic Reflectors staying close to each other  
 
with elements spacing of about 50lambda, where starts the Far Field of this array?  
 
in the far field of each parabolic antenna or in the far field of array?  
 
thanks in advance.

Typically 10*lambda for a single element or 10* the maximum physical dimension of the array (or even more), whichever is longer.  Some mm-wave antennas can have really distant far-fields.

In principal yes. The statement of madengr is what is used in general. You should use the total aperture dimension, not the center-center-distance between the dishes. There are small offsets due to illumination of the dish. Also end-fire arrays are a bit tricky.
 
Some definitions use the gain loss criterion as the far-field border.
Others use the side-lobe increase as the border. Also the mie region can be used as a definition, either its first crossing, or using the envelope. EMC guys use the first point where E and H fields are the same.
 
The gain criterion is the easiest for insight. Imagine a source really far into the farfield at bore-sight. This will generate a flat wave front, which will perfectly add up in the focus-point. When we move this source closer, the wave-front will have a circular shape, so it will not add up anymore perfectly in phase at the focus point. So coming closer to the dish will result in a gain loss at boresight.