I have flysky fs-ct6b/R6B tx/rx.
I connected a 4v battery directly on the rx and 3 servos.
Whenever I operate any one servo rx light start blinking and all servos starts to jitter (vibrating with a considerable magnitude) for 20-30 secondes then stops.it happens every time.

Check with different ESC

You may have answer here. http://www.rcindia.org/radios-and-receivers/what-is-current-capacity-of-flysky-receiver/msg295741/#new

Your receiver is rated for 5V. So if you connect a 4V battery it is low in the first place.
Whenever you operate a servo it draws considerable amount of current which may lead to voltage sag in the battery i.e decrease in battery terminal voltage. If this terminal voltage is below the brownout voltage of receiver then receiver will just reboot or restart.

remedy:
1. Since the receiver is rated for 5V+- 10~15% you can better plug a battery with that voltage.
2. Choose your battery such that it is capable of delivering required current for your application. This is the reason why many hobbyists choose liion or lipo.

Thanks for your reply!