Battery replacement with 18650 LiIon cells

I wanted to replace a 4x1,5 volts baby cells power arrangement in an old radio ITT teddy 4. New battery should be the ubiquitous 18650 cell. It offers about 4.2 volts, so I looked for a solution that does up conversion to about 6 volts, and that offers a USB connector to load the cell. Best would be to have also some battery protection (undervoltage protection, current overload protection). I found two interesting modules for further analysis.