# код, до меня
def forget_s(event):
    global button
    if button == 1:
        b1.place(x=50, y=70)
    elif button == 2:
        b2.place(x=150, y=70)
    elif button == 3:
        b3.place(x=250, y=70)
    elif button == 4:
        b4.place(x=350, y=70)
    if s.get() == 1:
        b1.place_forget()
        button = 1
    elif s.get() == 2:
        b2.place_forget()
        button = 2
    elif s.get() == 3:
        b3.place_forget()
        button = 3
    elif s.get() == 4:
        b4.place_forget()
        button = 4
    l.config(text="Кнопка скрыта:" + str(button))

# код, после меня
def forget_s(event):
    global button
    globals()['b' + str(button)].pack()
    button = s.get()
    globals()['b' + str(s.get())].pack_forget()
    l.config(text="Кнопка скрыта:" + str(button))