diff --git a/mu3hook/io4.c b/mu3hook/io4.c
new file mode 100644
index 0000000..2b9d820
--- /dev/null
+++ b/mu3hook/io4.c
@@ -0,0 +1,115 @@
+#include <windows.h>
+
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "board/io4.h"
+
+#include "mu3io/mu3io.h"
+
+#include "util/dprintf.h"
+
+static HRESULT mu3_io4_poll(void *ctx, struct io4_state *state);
+
+static const struct io4_ops mu3_io4_ops = {
+    .poll = mu3_io4_poll,
+};
+
+static int mu3_lever_pos;
+
+HRESULT mu3_io4_hook_init(void)
+{
+    HRESULT hr;
+
+    hr = io4_hook_init(&mu3_io4_ops, NULL);
+
+    if (FAILED(hr)) {
+        return hr;
+    }
+
+    return mu3_io_init();
+}
+
+static HRESULT mu3_io4_poll(void *ctx, struct io4_state *state)
+{
+    uint8_t opbtn;
+    uint8_t left;
+    uint8_t right;
+    int16_t lever;
+    HRESULT hr;
+
+    memset(state, 0, sizeof(*state));
+
+    hr = mu3_io_poll();
+
+    if (FAILED(hr)) {
+        return hr;
+    }
+
+    opbtn = 0;
+    left = 0;
+    right = 0;
+    lever = 0;
+
+    mu3_io_get_opbtns(&opbtn);
+    mu3_io_get_gamebtns(&left, &right);
+    mu3_io_get_lever(&lever);
+
+    if (opbtn & MU3_IO_OPBTN_TEST) {
+        state->buttons[0] |= IO4_BUTTON_TEST;
+    }
+
+    if (opbtn & MU3_IO_OPBTN_SERVICE) {
+        state->buttons[0] |= IO4_BUTTON_SERVICE;
+    }
+
+    if (left & MU3_IO_GAMEBTN_1) {
+        state->buttons[0] |= 1 << 0;
+    }
+
+    if (left & MU3_IO_GAMEBTN_2) {
+        state->buttons[0] |= 1 << 5;
+    }
+
+    if (left & MU3_IO_GAMEBTN_2) {
+        state->buttons[0] |= 1 << 4;
+    }
+
+    if (right & MU3_IO_GAMEBTN_1) {
+        state->buttons[0] |= 1 << 1;
+    }
+
+    if (right & MU3_IO_GAMEBTN_3) {
+        state->buttons[1] |= 1 << 0;
+    }
+
+    if (right & MU3_IO_GAMEBTN_3) {
+        state->buttons[0] |= 1 << 15;
+    }
+
+    if (left & MU3_IO_GAMEBTN_MENU) {
+        state->buttons[1] |= 1 << 14;
+    }
+
+    if (right & MU3_IO_GAMEBTN_MENU) {
+        state->buttons[0] |= 1 << 13;
+    }
+
+    if (!(left & MU3_IO_GAMEBTN_SIDE)) {
+        state->buttons[1] |= 1 << 15;   /* L-Side, active-low */
+    }
+
+    if (!(right & MU3_IO_GAMEBTN_SIDE)) {
+        state->buttons[0] |= 1 << 14;   /* R-Side, active-low */
+    }
+
+    /* Lever increases right-to-left, not left-to-right.
+
+       Use 0x7FFF as the center point instead of 0x8000; the latter would
+       overflow when the lever pos is INT16_MIN. */
+
+    state->adcs[0] = 0x7FFF - lever;
+
+    return S_OK;
+}
diff --git a/mu3hook/io4.h b/mu3hook/io4.h
new file mode 100644
index 0000000..e427d20
--- /dev/null
+++ b/mu3hook/io4.h
@@ -0,0 +1,5 @@
+#pragma once
+
+#include <windows.h>
+
+HRESULT mu3_io4_hook_init(void);