If you are using USB and do not have WiFi set up on the Spark, then it won’t have an IP.
You can configure WiFi from the Spark’s service page top right menu if you want to use WiFi instead of USB.
Short update: Andres measured that the actual voltage was 4.7V. Why the measurement reported by the software is wrong is still unknown, but it is unlikely to cause issues.
I’ll keep an eye on it.