I never owned Koss Pro 4A's but did own Koss Electrostatic headphones of the same era. They were a similar form factor and weight, and from me, were brutal to wear. It was them the convinced me that I really didn't like headphones despite the great resolution of these 'phones.

The Koss Electrostatics, BTW, were considered state of the art at the time, (as I recall -- long time ago) and cost me a then stupendous C$400, (maybe $2400 today). They had to be driven by a power amplfier, no phono jack for them.