They measured photon 1 because they wanted to prove correlation (it would be impossible to do without measuring it). If you believe they proved it, it works bot

They measured photon 1 because they wanted to prove correlation (it would be impossible to do without measuring it). If you believe they proved it, it works both ways. Hadn't they measured photon 1, our measurement of photon 4 would determine polarization of photon 1 a billion years ago. Entanglement works both ways, according to the theory.