Ok so I will call the son of Henry IV and Joan of Navarre, Prince Charles (after her father), he would have to be born between 1403 and 1413, lets say 1408.
So Prince Charles, would be given the titled, Charles of Lancaster, 1st Duke of York. He is five, when his father dies and his oldest half brother, Henry V takes the in 1413.
At the age of 16, he would be married off, most likely to just an English noblewoman, such as Elizabeth le Despenser or Cecily Neville.
His line will also affect France, as his blood line would be in line to the throne of the duchy of Brittany, meaning any male, alive in 1488, would come before OTL Anne, Duchess of Brittany.
In 1422, Charles would be able to help as Regent of England, while Henry VI is underage.
The War of the Roses, IOTL does not begin until 1455, when TTL Charles is 47, so if the war happens he will be able to support his nephew, Henry VI and would try and marry his daughter to him, so that the throne stays in the hands of loyal and trustworthy people.