import tensorflow as tf

x1 = tf.Variable(4)                    # Does dot work.
x2 = tf.Variable(4.)                   # Works.
x3 = tf.Variable(4.0)                  # Works.
x4 = tf.Variable(4, dtype=tf.float32)  # Works.

with tf.GradientTape(persistent=True) as tape:
    y1 = x1 ** 2
    y2 = x2 ** 2
    y3 = x3 ** 2
    y4 = x4 ** 2

dy_dx_1 = tape.gradient(y1, x1)
dy_dx_2 = tape.gradient(y2, x2)
dy_dx_3 = tape.gradient(y3, x3)
dy_dx_4 = tape.gradient(y4, x4)

del tape

f"{dy_dx_1}, {dy_dx_2}, {dy_dx_3}, {dy_dx_4}"

None, 8.0, 8.0, 8.0