Why do we make T4 if we only use T3?
It sounds like what you read and what your professor mentioned are different types of causation. 

T4 is the primary form produced by the thyroid; T3 is metabolized from T4 in other tissues and more strongly activates thyroid hormone receptors. Metabolism from T4 to T3 then allows tissues to have control over how much active thyroid hormone there is - multiple levels of control are very common to hormones, and other signaling pathways as well.