diff --git a/ttakahashi/chapter04/q01.py b/ttakahashi/chapter04/q01.py
new file mode 100644
index 0000000..110b191
--- /dev/null
+++ b/ttakahashi/chapter04/q01.py
@@ -0,0 +1,26 @@
+import numpy as np
+
+
+def pad(x, L, S):
+    x_pad = np.pad(x, [L - S, L - S])
+    re = np.mod(x_pad.size, S)
+    if re != 0:
+        x_pad = np.pad(x_pad, [0, S - re])
+    return x_pad
+
+
+def main():
+    L = 4
+    S = 3
+
+    x = np.ones(8)
+    x_pad = pad(x, L, S)
+
+    print(f"x: {x}")
+    print(f"x_pad: {x_pad}")
+
+
+if __name__ == "__main__":
+    main()
+
+    print("Finished")
diff --git a/ttakahashi/chapter04/q02.py b/ttakahashi/chapter04/q02.py
new file mode 100644
index 0000000..e2c8f79
--- /dev/null
+++ b/ttakahashi/chapter04/q02.py
@@ -0,0 +1,29 @@
+import numpy as np
+from q01 import pad
+
+
+def frame_div(x, L, S):
+    x_pad = pad(x, L, S)
+    T = int(np.floor((x_pad.size - L) / S)) + 1
+    x_t = np.array([x_pad[t * S : t * S + L] for t in range(T)])
+    return x_t
+
+
+def main():
+    L = 4
+    S = 3
+
+    x = np.ones(8)
+    x_pad = pad(x, L, S)
+    x_t = frame_div(x, L, S)
+
+    print(f"x: {x}")
+    print(f"x_pad: {x_pad}")
+    print(f"x_t: {x_t}")
+    print(f"x_t.shape: {x_t.shape}")
+
+
+if __name__ == "__main__":
+    main()
+
+    print("Finished")
diff --git a/ttakahashi/chapter04/q03.py b/ttakahashi/chapter04/q03.py
new file mode 100644
index 0000000..9c3c494
--- /dev/null
+++ b/ttakahashi/chapter04/q03.py
@@ -0,0 +1,34 @@
+import numpy as np
+from q01 import pad
+from q02 import frame_div
+
+
+def stft(x, L, S, wnd):
+    x_t = frame_div(x, L, S)
+    T = len(x_t)
+    X = np.array([np.fft.rfft(x_t[t] * wnd) for t in range(T)], dtype="complex")
+    return X.T
+
+
+def main():
+    L = 4
+    S = 3
+
+    x = np.ones(8)
+    wnd = np.hamming(L)
+    x_pad = pad(x, L, S)
+    x_t = frame_div(x, L, S)
+    x_stft = stft(x, L, S, wnd)
+
+    print(f"x: {x}")
+    print(f"x_pad: {x_pad}")
+    print(f"x_t: {x_t}")
+    print(f"x_t.shape: {x_t.shape}")
+    print(f"x_stft: {x_stft}")
+    print(f"x_stft.shape: {x_stft.shape}")
+
+
+if __name__ == "__main__":
+    main()
+
+    print("Finished")
diff --git a/ttakahashi/chapter04/q04.py b/ttakahashi/chapter04/q04.py
new file mode 100644
index 0000000..83ed920
--- /dev/null
+++ b/ttakahashi/chapter04/q04.py
@@ -0,0 +1,80 @@
+from pathlib import Path
+from typing import Union
+import numpy as np
+import matplotlib.pyplot as plt
+from q03 import stft
+
+
+config = {
+    "figure.subplot.bottom": 0.15,
+    "figure.subplot.left": 0.15,
+    "figure.figsize": [10, 8],
+    "font.size": 24,
+    "font.family": "Times New Roman",
+    "mathtext.fontset": "cm",
+    "pdf.fonttype": 42,
+    "legend.borderaxespad": 1,
+    "lines.linewidth": 2,
+    "savefig.transparent": True,
+}
+
+
+LABELS = {
+    0: ("viridis", "Power spectrogram"),
+    1: ("viridis", "Phase spectrogram"),
+    2: (plt.cm.Reds, "Cosine of phase spectrogram"),
+    3: (plt.cm.Blues, "Sine of hase spectrogram"),
+}
+
+
+def main(output_dir: Union[Path, str]):
+    out_p = Path(output_dir)
+
+    A = 1  # amplitude
+    f = 440  # frequency (Hz)
+    sec = 0.1  # time (sec)
+    sr = 16000  # sampling rate (Hz)
+
+    t = np.arange(sec * sr) / sr
+    y = A * np.sin(2 * np.pi * f * t)
+
+    L = 1000
+    S = 500
+    wnd = np.hamming(L)
+    y_stft = stft(y, L, S, wnd)
+
+    print(f"y_stft.shape: {y_stft.shape}")
+
+    spec = [np.abs(y_stft), np.angle(y_stft), np.cos(np.angle(y_stft)), np.sin(np.angle(y_stft))]
+
+    plt.rcParams.update(config)
+    fig, ax = plt.subplots(4, 1)
+    for i in range(len(spec)):
+        # ax[i].pcolormesh(spec[i])
+        ax[i].imshow(
+            spec[i],
+            cmap=LABELS[i][0],
+            interpolation="nearest",
+            aspect="auto",
+            origin="lower",
+            extent=[0, y_stft.shape[1], 0, y_stft.shape[0]],
+        )
+        ax[i].set_title(LABELS[i][1])
+        ax[i].set_rasterized(True)
+    fig.supxlabel("Frame")
+    fig.supylabel("Frequency (Hz)")
+    plt.tight_layout()
+    fig.subplots_adjust(left=0.13, right=1 - 0.13, bottom=0.11, top=1 - 0.11, hspace=0.8)
+    plt.savefig(out_p / "04.pdf", dpi=300)
+    plt.clf()
+    plt.close()
+
+
+if __name__ == "__main__":
+    out_p = Path.cwd() / "outputs"
+    if not out_p.exists():
+        out_p.mkdir(parents=True)
+
+    main(out_p)
+
+    print("Finished")
diff --git a/ttakahashi/chapter04/q05.py b/ttakahashi/chapter04/q05.py
new file mode 100644
index 0000000..44cc5cf
--- /dev/null
+++ b/ttakahashi/chapter04/q05.py
@@ -0,0 +1,62 @@
+from pathlib import Path
+from typing import Union
+import numpy as np
+import matplotlib.pyplot as plt
+
+
+config = {
+    "figure.subplot.bottom": 0.15,
+    "figure.subplot.left": 0.15,
+    "figure.figsize": [10, 8],
+    "font.size": 24,
+    "font.family": "Times New Roman",
+    "mathtext.fontset": "cm",
+    "pdf.fonttype": 42,
+    "legend.borderaxespad": 1,
+    "lines.linewidth": 2,
+    "savefig.transparent": True,
+}
+
+
+def comp_wnd(wnd, S):
+    L = wnd.size
+    Q = int(np.floor(L / S))
+    cwnd = np.empty(L, dtype=np.float64)
+    for i in range(L):
+        k = i - (Q - 1) * S
+        if k < 0:
+            k = 0
+        cwnd[i] = wnd[i] / np.sum(wnd[k : i + (Q - 1) * S] ** 2)
+    return cwnd
+
+
+def main(output_dir: Union[Path, str]):
+    out_p = Path(output_dir)
+
+    L = 1000
+    S = 500
+    wnd = np.hamming(L)
+    cwnd = comp_wnd(wnd, S)
+
+    plt.rcParams.update(config)
+    fig, ax = plt.subplots(2, 1)
+    ax[0].plot(wnd)
+    ax[0].grid()
+    ax[0].set_title("Hamming window")
+    ax[1].plot(cwnd)
+    ax[1].set_title("Composite window")
+    ax[1].grid()
+    plt.tight_layout()
+    plt.savefig(out_p / "05.pdf")
+    plt.clf()
+    plt.close()
+
+
+if __name__ == "__main__":
+    out_p = Path.cwd() / "outputs"
+    if not out_p.exists():
+        out_p.mkdir(parents=True)
+
+    main(out_p)
+
+    print("Finished")