As it says on the tin:
Ferdinand IV, was king of Hungary & Bohemia and probable future Holy Roman Emperor died at the age of 20 in 1654 of smallpox. Because of this Leopold I was pulled from his destiny in the church and became HRE Ferdinand III's heir apparent.
One of the few effects I could see of Ferdinand IV surviving is that Maria Teresa of Spain would marry that cousin and thus be rendered unavailable for Louis XIV, who'll marry either Minette or Margherita Violante of Savoy instead.
Also, Leopold was said to be bigoted/overly pious due to his being raised for the church. For instance, he'd sign death warrants, and then go to mass to pray for the souls of those he'd just condemned.
Thoughts?