Was the original Python model Keras saved model (.h5 file) or regular TF savedModel (typically .pb file)?
For Keras saved model you would use:
tf.loadLayersModel(MODEL_URL);

For TF SavedModel you would use:
tf.loadGraphModel(MODEL_URL);

Looking at your error message though in the first instance it seems your passing a tensor of the wrong shape. Check the tensor of the data you are sending into the model and figure out why it is 4 instead of 3 in that last dimension. What image did you load? PNG? What exactly is your input_tensor? Do you have this on a Glitch or Codpen somewhere for me to see running?