I’m adding the BC127 to an existing design that has 4V on board but no 3.3V. If I supply 4V to VBAT, will the 3V3_USB pin provide a regulated 3.3v output that I can use to supply the VDD_PADS pin?
I wouldn’t do that, personally. 3V3_USB is an output, but I don’t see the max current support listed anywhere.
Have you considered a Torex XC62010 LDO? It only requires two 1uF ceramic caps and it would take care of providing you with the 3.3V to run the BC127.