consfued about a fomula in the paper

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Hi I'm trying to learn how DPOP works but got stuck at the Derivation of DPO part (the provement of DPOP's motivation)
In specific, the equation 4 in Appendix B.1 Derivation for DPO.
Could you help elaborate a little on how equation 4 generates or could you give more directions on the references?

Really appreciate it!