Three came after two

Eileen’s two-year-old great-grandson was excited about having his birthday in a few days. When asked how old he would be, he always said he would be four and held up four fingers.
His mother tried to explain that he would be three, that three came after two, but he wasn’t convinced.
He told her that he had to be four because when he tried to hold up three fingers, the fourth came up too.

Serious problem

Ohh, I need..