I'd read in an interview that he said that he didn't care if he was known as Willem IV or Willem Alexander, he would leave that up to the historians to work out. He said he would always refer to himself as Willem Alexander
"Despite the serious topics covered in the interview, he even found time for a joke. Asked if he would reign as Willem IV, the royal said he would be known as King Willem-Alexander. Referring to the way cows are named, he quipped dryly: "Willem 4 stands next to Bertha 38 in the meadow".