4 in 1
A '4 in 1' combines much of the electronics required on a helicopter into one unit, often found on ready to fly models. A 4 in 1 contains the receiver, gyro and two electronic speed controllers (it also contains a revolution mixer and a battery eliminator circuit, either of which could be counted in place of one of the ESCs).
Sometimes '3 in 1' is used to refer to a '4 in 1' with a separate receiver. For example, the Esky 4 in 1 used in their lower end ready to fly models can be broken into two halves, one containing the receiver, and one containing the rest of the electronics. This allows the receiver to be replaced easily to talk to other radio systems, such as 2,4GHz systems.
A 4 in 1 is a lightweight and compact solution to flight electronics, but they are very inflexible, as individual components cannot easily be swapped out. Changing from a brushed motor to a brushless motor for example is quite difficult, and leaves the original brushed motor controller redundant on the aircraft. Changing the gyro would also be hard to impossible to do well.
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