If power factor is unknown or cannot be determined, it is common to assume pf=0.8.
So, for your case
kw = 4.0
kva = kw/pf = 4.0/0.8 = 5.0
Note, this assumes the circuit your dealing with is predominatly resistive & inductive. For a resistive & inductive circuit the power factor is termed a "lagging" power factor. On the other hand, if the circuit is predominately capacitive, power factor is termed a "leading" power factor.
Hope this sheds some electrons on the situation for you ;-)
Rich
