For example, if piezo disc has lowest resonant frequency of 4kHz. At that frequency, would the electrical impedance be at its maximum or minimum?
It's both. Just like regular crystals, piezo exhibits resonance and anti-resonance within a small part of the spectrum. See this for example. At one resonant point it will act like a series resonant circuit and appear like a short circuit and at the other point it will be a parallel (high impedance) circuit: -
See also this tutorial from Maxim - it contains the basic electrical model of a pizeo and note its similarity with a regular crystal: -