Steve used a spirometer to analyze his respiratory capacities. He found the following results: a) after normal inspiration, a normal expiration was 500 ml
b) following a normal expiration, he was able to expel an additional 1000 ml
c) taking as deep a breath as possible, then forcefully exhaling all the air possible, he yielded an output of 4500 ml.

On the outcome of these measurements, what is Steve’s inspiratory reserve?