Complex sentence: Prem had done 4 movies before Woh Kaun Thi, a box-office hit of 1964, had released.It was on sets of Woh Kaun Thi which had Manoj Kumar as lead hero where Prem met Manoj for the very first time.
Split into simpler sentences: Prem had done four movies before "Woh Kaun Thi?", a box-office hit of 1964, had released. On the sets of "Woh Kaun Thi?" which had Manoj Kumar as lead hero Prem met Manoj for the first time.