While retraining a pretrained model, getting: ValueError: Input 0 is incompatible with layer flatten_1: expected min_ndim=3, found ndim=2
The reason for this error is that you are trying to flatten an already flat layer. The output of your model is (batch_size, 4), which cannot be flattened further. To simply fix the error remove the flatten layer from your code.

However, when fine-tuning a pretrained model, you should first remove the top layers of that model before adding your own. The reason is that these layers are trained for classification on a task different than yours.

If I were you I'd drop the last two layers of your pretrained model:

# code same as before ...

x = model.layers[-3].output  # Flatten layer output

# don't add flatten again

for fc in fc_layers:
    x = Dense(fc, activation='relu')(x)
    x = Dropout(dropout)(x)

predictions = Dense(num_classes, activation='softmax')(x)

finetune_model = Model(inputs=model.input, outputs=predictions)