Official GTK4 example 4 (https://docs.gtk.org/gtk4/getting_started.html)

builder.ui

<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <object id="window" class="GtkWindow">
    <property name="title">Grid</property>
    <child>  
      <object id="grid" class="GtkGrid">
          <child>
            <object id="button1" class="GtkButton">
              <property name="label">Button1</property>
      	    <layout>
      	      <property name="column">0</property>
      	      <property name="row">0</property>
      	    </layout>
            </object>
          </child>
          <child>
      	  <object id="button2" class="GtkButton">
      	    <property name="label">Button2</property>
      	    <layout>
      	      <property name="column">1</property>
      	      <property name="row">0</property>
      	    </layout>
            </object>
          </child>
          <child>
          	<object id="quit" class="GtkButton">
      	  <property name="label">Quit</property>
      	  <layout>
      	    <property name="column">0</property>
      	    <property name="row">1</property>
      	    <property name="column-span">2</property>
      	  </layout>
          	</object>
          </child>
      </object>
    </child>
  </object>
</interface>

example-4.c

/** example-4.c	
 *
 * COMPILE
 *
 * gcc `pkg-config --cflags gtk4` -o example-4 example-4.c `pkg-config --libs gtk4`
 *
 * EXAMPLE
 *
 * ./example-4
 *
 * SOURCE
 *
 * https://docs.gtk.org/gtk4/getting_started.html
 */

#include <gtk/gtk.h>
#include <glib/gstdio.h>

#if GLIB_CHECK_VERSION(2, 74, 0)
static int app_flags = G_APPLICATION_DEFAULT_FLAGS;
#else
static int app_flags = G_APPLICATION_FLAGS_NONE;
#endif

static void
print_hello (GtkWidget *widget,
             gpointer data)
{
  g_print ("Hello World\n");
}

static void
quit_cb (GtkWindow *window)
{
  gtk_window_close (window);
}

static void
activate (GtkApplication *app,
          gpointer user_data)
{
  GtkBuilder *builder = gtk_builder_new ();
  gtk_builder_add_from_file (builder, "builder.ui", NULL);

  GObject *window = gtk_builder_get_object (builder, "window");
  gtk_window_set_application (GTK_WINDOW (window), app);

  GObject *button = gtk_builder_get_object (builder, "button1");
  g_signal_connect (button, "clicked", G_CALLBACK (print_hello), NULL);

  button = gtk_builder_get_object(builder, "button2");
  g_signal_connect (button, "clicked", G_CALLBACK (print_hello), NULL);

  button = gtk_builder_get_object (builder, "quit");
  g_signal_connect_swapped (button, "clicked", G_CALLBACK (quit_cb), window);

  gtk_widget_show (GTK_WIDGET(window));

  g_object_unref (builder);
}

int
main (int argc,
      char *argv[])
{
#ifdef GTK_SRCDIR
  g_chdir (GTK_SRCDIR);
#endif

  GtkApplication *app = gtk_application_new ("org.gtk.example", app_flags);
  g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);

  int status = g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);

  return status;
}