Captain Inzamam is recovering from a finger injury, which he sustained in the third game in Lahore, while Younis and Yousuf have opted out due to personal problems.
Younis has yet to join the team after attending the funeral of his elder brother who died in Germany last week while Yousuf is with his ailing wife.
Yousuf also missed the second match in Faisalabad and Younis was not available for the third game.
''Razzaq, by virtue of being the most senior player in the side, should lead the team,'' Saleem Altaf, the Pakistan Cricket Board's director of operations, told Reuters.
''We will make a formal announcement later today,'' he said.
Razzaq has played in 46 tests and 222 one-day internationals.
Altaf said the selectors did not want to take a risk with Inzamam.
''His finger is healing as it has got three stitches and is still bruised. With the South Africa tour round the corner we don't want any problems with his fitness,'' he said.
''The selectors will have the opportunity to try out some of the younger players.'' It would be the first time since Younis made his debut in February 2000 that the Pakistan team would be without three of their most senior batsmen.
Pakistan, who lead the five-match series 2-0, have called up Faisal Iqbal and Shahid Afridi into the squad. The first match in Rawalpindi was washed out.