Hey! Let me get a bit nerdy and explain a bit about the IP rotation mechanism in 4G proxies (and mobile proxies in general).
If you have a mobile phone, every time it connects to a cell tower of your mobile operator, it gets an IP address. Most operators use dynamic IPs, meaning each time you connect to the tower, your device gets a new IP. So, simply reconnecting to the tower (like turning airplane mode on and off) can change your IP.
Now, about your question. Each cell tower has a certain pool of IP addresses. How big is this pool? It varies a lot depending on the tower. In practice, it ranges from hundreds to a few thousand IPs (talking about IPv4 here, and even the IPv6 standard isn't likely to offer millions). So, the claim of "millions of IPs at a single mobile proxy server" is only possible if your provider combines many mobile devices, ideally from different locations. And having simultaneously 100 different mobile IPs, wow, buddy, that can be quite pricey.