How does Theseus's current decision regarding Hermia and Lysander contradict his earlier statement?This is regarding Act 4.

I assume that you are talking about the part in Act IV, Scene 1,.  In that scene, Theseus tells Egeus that he is going to override Egeus's decision about who his daughter will marry.  Egeus had told Hermia she had to marry Demetrius and Theseus had backed him up.  Now Theseus is contradicting his earlier statement.

Back in Act I, Scene 1, Theseus told Hermia that she had to take her father's word like the word of God.  She had to do what he said or be executed (or enter a convent).  But now he is going to overrule Egeus and order that Hermia and Lysander and Demetrius and Helena all be married at the same time he marries Hippolyta.