qmk/keyboards/vial_example/vial_atmega32u4/keymaps/default/keymap.c

#include QMK_KEYBOARD_H

const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
    [0] = LAYOUT(
        KC_1, KC_2,
        KC_3, KC_4
    ),

    [1] = LAYOUT(
        KC_A, KC_B,
        KC_C, KC_D
    ),

    [2] = LAYOUT(
        KC_TRNS, KC_TRNS,
        KC_TRNS, KC_TRNS
    ),

    [3] = LAYOUT(
        KC_TRNS, KC_TRNS,
        KC_TRNS, KC_TRNS
    )
};