Guide to joining with Bedrock
All consoles can join third-party servers - including Geyser servers - with workarounds. Xbox One and Nintendo Switch systems can join third-party servers using a third-party program called BedrockConnect. On PS4, other methods are available for use.
Unfortunately at this time, PS4 does not have any external server support. However, there are ways around this by emulating a LAN game on your network on another device.
Note that this method will also work with any other Bedrock platform other than Nintendo Switch.
Using a PC with a PlayStation 4
If you have a PC, you can use Phantom.
Using an Android Device with a PlayStation 4
If you have an Android device, you have several options:
Using an iOS device with a PlayStation 4
If you have an iOS 14+ device, you can use BedrockTogether.