There are two IP address versions: IPv4 and IPv6.
IPv4 is an Internet protocol with 32-bit address space length, containing approximately 4.3 billion addresses. Today this IP address version is considered standard, and is supported by all web resources. However, due to free address exhaustion, this version is becoming outdated and gradually replaced by IPv6.
IPv6 is the newer version of this protocol, with the address itself 4 times longer. The amount of IPv6 addresses is 1028
bigger than that of IPv4, and IPv6 addresses are more secure due to authentication and encryption mechanisms. IPv6 is most commonly used for mobile access.
You can see the IPv6 adoption growth rate using this Google service
Proxy providers offer both IP address formats, and your choice depends on your proxy usage aims. It is harder to find free IPv4 addresses due to general shortage, and this is why renting an IPv6 address usually costs less. However, not all web resources support IPv6, and some proxy servers might work with the newer IP version incorrectly.