Constant PLUGIN_API_VERSION_V4 

pub const PLUGIN_API_VERSION_V4: u32 = 4;

API version v4: adds set_log_sink so a plugin’s log records reach the hub’s log. A plugin .so has its own copy of the log crate, so without the sink its records go nowhere. define_plugin! installs the sink unconditionally — plugins built against v4 forward their logs with no source change. v3-and-older plugins keep loading on v4 hubs (the hub gates on api_version >= 4 and skips the install); v4 plugins keep loading on older hubs (which ignore the field, so their logs stay silent there).