Inzamam has been advised to rest for three days after he required three stitches to a finger injured while fielding in the third one-day international on Sunday while Yousuf's wife is ill.
Pakistan won by seven wickets to take an unbeatable 2-0 lead in the five-match series. The first match was abandoned.
Batsman Younis Khan also missed the match after attending the burial rites of his elder brother who died in Germany last week. Family members said he was unlikely to play in the fourth match as well.
"We have called up Afridi as Yousuf will not play the fourth match as his wife is very ill. Afridi has done well in domestic cricket this week and the selectorsfelt he was the natural choice," a Pakistan Cricket Board spokesman said.
Yousufalso missed the second game in Faisalabad which Pakistan won by two wickets due to hi wife's illness.
Afridi who was dropped for the series after scoring just 114 runs in his last 17 games made a century and a half-centuryas well as taking four wickets in a first class match for his department at the weekend.
Inzamamtold reporters doctors had advised him to rest the finger for three days.
"Itis nothing serious but they have said I should only start practising after I feel comfortable.I am not certain if I will be able to play the fourth game," he said.
Pakistan squad: Inzamam-ul-Haq (captain),Imran Farhat, Mohammad Hafeez, Yasir Hameed, Faisal Iqbal, Younis Khan, Kamran Akmal, Abdul Razzaq, Umar Gul, Rana Naved, Rao Iftikhar, Mohammad Sami, Danish Kaneria, Abdul Rehman.