Please refer to page 10 of CX3 datasheet. The link to the same is given below:
The minimum required VBUS for CX3 is 4V from the datasheet. But for stable enumeration, voltage on VBUS should be above 4.1V as mentioned in AN70707. So 4.2V on VBUS should be enough provided that the voltage never go below 4.1V during operation. To be on a safer side, we recommend to keep the VBUS at 5V. In addition to this, you need to follow the power up sequence correctly as shown in Figure 4 of the datasheet.