As you described, there is no danger to your amp or speakers, or talk box. you will notice a diminished response (lower volume) with the box in series (8 0hms + 4 ohms = 12 ohms resistance, or more accurately, impedance). In parallel, you have 8 + 4 divided by 2, or 6 ohms, during talk box operation. You'll be fine!
Lower impedance hurts the amplifier, not the speakers. I have always considered running less than a 4 ohm load to constitute amplifier abuse. Excessive current (wattage) is what hurts the components.
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