Here's a very nice video (see it in HD) of timelapses captured at the atacama desert. In the beginning of the video you can see that 4 telescopes move synchronously. Could anyone explain why? I always thought that each telescope was meant for observing different events, but it doesn't seem to be the case here.
Interferometers work by combining the signal recorded by many individual dishes to reconstruct the original image. To make this work all the dishes have to be pointed towards the same object, and that's why the video shows them moving in sync.
What you are seeing is the ALMA http://www.eso.org/public/teles-instr/alma/ which is an array of radio telescopes whose data can be combined to simulate the performance of a much bigger radio telescope.
So the reason they're moving in synchrony is that they are all looking at the same target.
All the answers serve very good explaining why all the telescopes are synced. The only question which remains is that why we are using 4 telescopes or any number of telescopes for that matter.
Actually, the quality of data collected increases with the diameter of the telescope. So, they place many telescopes in an array to increase the diameter of the telescope which is much easier to operate then. Secondly, it is also used for noise subtraction i.e. if the signals received by all the telescopes are not same they refer to a noise or some background which can be then easily subtracted from the data which would not have been possible if a single large telescope is used.