Perhaps you should add the directory containing `gtk+-4.0.pc' to the PKG_CONFIG_PATH environment variable

This error message usually occurs when the `pkg-config` utility is unable to find the `gtk+-4.0.pc` file, which is required to build GTK applications. To fix this issue, you need to add the directory containing the `gtk+-4.0.pc` file to the `PKG_CONFIG_PATH` environment variable.

Assuming you have installed GTK+ 4.0 in the default location, you can run the following command to add the directory to the `PKG_CONFIG_PATH` variable:

export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:$PKG_CONFIG_PATH

This command adds the `/usr/local/lib/pkgconfig` directory to the `PKG_CONFIG_PATH` variable. If you have installed GTK+ 4.0 in a different location, replace `/usr/local/lib/pkgconfig` with the path to the directory containing the `gtk+-4.0.pc` file.

After setting the `PKG_CONFIG_PATH` variable, try running the `pkg-config` command again to see if it can find the `gtk+-4.0.pc` file. If it still can't find the file, make sure that the directory containing the file is included in the system's `PATH` variable.