I want a 4 channel receiver for my flysky th9x which is upgraded to er9x.
Due to size restrictions i want only 4 channel receiver.

Thank you in advance

There is one 4Ch Rx,  FS-IR4. It's a surface Rx, be sure to test the working range of the Rx before using.
The FlySky radios use AFDHS modulation, so look around for 4Ch AFDHS receivers, they should work. I use a 3Ch Turnigy GTX3 surface Rx with the FS-TH9X without issues.

Thank you for your answer brother
Sorry to ask but can you give me a link ??

http://www.ebay.com/itm/Flysky-FS-iR4-4CH-2-4Ghz-AFHDS2-RC-Receiver-For-FS-iT4-FS-IR4-/181040505529

Mind you, this is a Rx made for the FS-IT4 surface telemetry radio. Although, the modulation is same as the FS-TH9X, I cannot assure you that it will work. I do not know what else has to be taken into consideration, besides signal modulation.