There is no internal voltage regulator on the RAK3172.
Although you can make it work at 4.7V, it is a matter of time before you see negative side effects of operating it beyond its operating range. There is no over voltage cutoff on the module that will disallow you to use it beyond 3.6V. However, if you use it beyond that, there will be cracks developing inside the module since it is above what the semiconductor expects.