I would say that is reasonable timeframe.
I land at T4 at least once a month
Admittedly I do not have bags to collect but it usually takes about 20 /30 mins to get off the plane, through passport control before I jump on the tube.
T4 to King’s Cross will be 1 hour 20 mins on the tube.
The lizzy line is much a quicker way to get in to town but will involve changing unlike the Piccadilly line.