You can connect the 4.2V supply to the VUSB pin on the Boron. Maximum input voltage is 17 VDC.
You may also need to lower the maximum charge current, as in the post below. It's for the Tracker One but the same principle applies to the Boron, just different voltages.
The reason is that when the input voltage exceeds the minimum charge voltage, charging will begin at 900 mA. For solar, in particular, this can cause the voltage to collapse, which causes charging to stop, and then the voltage recovers, and then it repeats this over and over. Limiting the charge current can help prevent this with smaller panels.