Calibrating Quattro ESC with Pixhawk

Hi, I am currently working on an autonomous project for college and have run into some issues with the Quattro 30A 4 in 1 ESC.
I’m running a Pixhawk with the Quattro 4 in 1 ESC and DYS 700kv motors and using a spectrum DX7s and AR8000 receiver.
The problem is that motor 4 wont spin up until I’m at half throttle on the Tx and I’m struggling to work out why. I have plugged the motor itself into one of the other ESC outputs and all works fine so I can assume the motor is okay.
I tried plugging the other ESC inputs into the channel 4 from the Pixhawk and they all worked as expected so I would have thought it safe to assume the Pixhawk is outputting the right signals.
I have tried recalibrating the ESC a number of times, both via the Pixhawk and the ESC by ESC method with no joy. Although, I have noticed that when entering calibration mode the ESC beeps differently to the 3DR video on ESC calibration. When I plug in the ESC with the throttle high for calibration mode it beeps continuously at roughly one second intervals - is this normal? Other than this, calibration mode is identical to the 3DR video and appears to work perfectly for motors 1, 2 and 3.
The Pixhawk arms everything just fine as well, the only problem is Motor 4 not spooling up until half throttle. Any ideas as to how to solve this bearing in mind none of the electronics on this set up have flown yet.
Thank you very much for reading through all that and thank you in advance for your help.

It does sound like you have not managed to complete the ESC calibration correctly.

It might be best to perform a manual ESC calibration by connecting the ESC’s directly to your R/C receiver using a using a ESC throttle hub. However since you have tried the one by one method it might be worth trying the motor 4 ESC again to check as maybe it did not save correctly.

Otherwise the last option is to get yourself a USB linker board so that you can connect your ESC to the PC and manually set the ESC calibration range for each one. It also allows you to check all the other ESC settings are the same between all 4 ESC’s

Thank you very much for you help, much appreciated. I will give these a go and hopefully it will all work out. Thanks!