rumqttd.toml: what is v4 and v5?

[boxofrox]

What v4 and v5 represent in the rumqttd.toml is unclear from the readme's and the toml example.

If these represented mqtt protocol selections, I'd expect v3 and v5, since I can find no mention of an mqtt v4 spec ever being released.

I thought maybe these were version numbers for config options, but I can't identify what options v5 has that v4 doesn't.

[detoxifiedplant]

The MQTT specification v3.1.1 is considered as v4,
while specification v5.0 is considered as v5

https://github.com/mqtt/mqtt.org/wiki/Differences-between-3.1.1-and-5.0