Each of the four individual telescopes in the ESA's Very Large Telescope are, by themselves, very capable. But by working together, they're supposed to be able to link up to effectively form one single massive telescope, and for the first time, this has actually worked.
The European Southern Observatory has, in the past, linked the two farthest apart telescopes in the VLT array together to form a virtual telescope with a mirror 130 meters wide. Adding in the two additional telescopes won't make this virtual mirror any larger, but it will improve the resolution of the instrument along with its ability to see things that are dimmer, farther away, or both: the "zooming" capability increases by a factor of 20 when all four telescopes team up.
Getting four individual telescopes to act like one single giant telescope isn't easy. Each beam of light gets piped from the mirror of each telescope underground through a series of tubes and mirrors and instruments to make sure that the four light beams stay synced up, which is tricky because the telescopes are all at different distances from each other. Furthermore, the problem with bouncing light around this much is that every bounce sucks a little bit of brightness out, and by the time a beam of light makes it all the way through the system (requiring some 32 bounces), about 95% of it is has been absorbed by the mirrors.
Even with this amount of dimming, the resolution offered by all four telescopes allows astronomers to measure the separation between stars in triple systems, and look closely at young stars surrounded by protoplanetary discs that will one day form solar systems. In the words of one VLT astronomer, "now, the zoo of objects accessible to us will be much bigger." The final checkout on the quad-telescope linkup was completed last week, meaning that scientific imaging on that zoo of objects can now officially begin.