My circuit is detecting 24Vac/dc signals with an opto isolator at the input. HCPL354 is the opto isolator and I am adding 4.4Kohms resistance in series to limit the current. When I make the datasheet for the final product, what value should be specified as Input impedance ? Is it 4.4Kohms itself ?
Impedance is only a useful specification when the input is an actual lumped impedance (potentially complex), largely constant across a range of frequencies. Yours is a nonlinear active circuit, so there's no single "impedance" spec that would be helpful to the user, even if theoretically you could hook up the input to a network analyzer and determine the input impedance across a range of frequencies.
You should specify that the input is an LED optocoupler input with a forward voltage of X Volts at Y mA, with a series resistance of 4.4k. The value of Y should be chosen to be helpful, i.e. it should be a current that would be expected in typical use.
You should also specify the minimum input current to guarantee activation of the optocoupler, including aging. I.e. if you say that the expected product life is 5000 hrs with all inputs on, then look in the optocoupler datasheet for CTR vs. time curve, and specify accordingly.
When making datasheets, it's a good idea to be actually helpful, and not merely produce specs that "look standard" but are in fact useless.