due to the way the panda 4x4 systems is designed you can tow it when the 4x4 system is not engaged (lever all the way to the floor). it is completely disconnected from the front gearbox when its like that. the risk would be the same as towing a standard rear wheel drive vehicle in neutral.
to the 4x4 system, normal driving of the car in 2wd mode has the same affect as if it was being towed. the system is only being moved by the wheels turning, not from the engine.
old mk1 and mk2 fiat pandas are not "true" 4x4s.. they just have a take off point from the front diff that can drive the rear wheels when enabled. an easy way to tell if its a "true" 4x4 is if the rear wheels are the ones that are constantly driven by a transfer box. the panda like what we have do not have a transfer box, just a dog type clutch that enables the rear axle.
every part of the system is lubricated via its own oil and no stress can incur from towing.
i am currently building a car that will be able to be towed (thankfully as i am building it) that will be a true 4x4. it will have 3 differentials. one on each axle and one in the transfer box (most 4x4s do not have the 3rd diff just a dog clutch). when all 3 diffs are unlocked (its normal state) no stresses will build up in any of the drivetrain.
hope this helps!