IPv4 is an Internet Protocol version that uses 32-bit addresses, commonly displayed as four numbers separated by dots. Each written number is between 0 and 255. IPv4 addresses can identify different kinds of networks; not every IPv4 address is a public internet address.
A practical example
The address 192.0.2.10 is an example reserved for documentation. An address you see inside your device settings can differ from the address reported by an external IP-checking service. They may describe different points along the same connection.
What to check
When comparing a connection before and after a VPN, record which tool you used and whether it reports IPv4 or IPv6. Repeat the same test rather than comparing unrelated screens. If one screen shows a local address and another shows a public address, first establish what each screen measures.
What it does not tell you
IPv4 is an addressing format, not a security rating. A change in one reported address does not prove that every app is routed through a VPN, and keeping the same local Wi-Fi address does not by itself show that the VPN failed.
Read the technical reference for the underlying definition. For a connection that is failing now, use the troubleshooting guide. To choose an app, follow the VPN selection worksheet.