Notice the servo wire in my left hand on the pic, I am pressing the leads from the voltmeter onto the servo connector that feeds voltage from the BEC which should be around 6.0V for most BEC's on the market.

Mushroomed gave some good suggestions to bypass the BEC, another option is to draw power from the #2 cell on your balance port to get unfiltered 8.4V with a fresh charge:
How to wire HV Servo from 2S LiPo

You can use this bypass to the Ch2 port on the Rx, I am showing a 2S-JST plug in the pic, but the same can be done with a 4S-JST, use a voltmeter to double check the voltage is 8.4V beforehand, assuming your Rx is rated for HV @ 8.4V:



If your Rx can only handle 6V, then I would get a UBEC like this instead:
https://www.ebay.com/itm/RC-UBEC-5V-...sAAOSwG8tcjzni