Colombo: Umar Akmal hit an unbeaten 102 off 72 balls as Pakistan thrashed Sri Lanka by 146 runs on Friday.
Umar's maiden century helped Pakistan to score 321-5 after the tourists elected to bat in the day-night match at the Premadasa stadium.
Pacerr Rao Iftikhar then grabbed 5-30 as Sri Lankans were dismissed for 175 in 36.1 overs, handing Pakistan a badly-needed win after they were beaten 2-0 in the Test series and lost the first three One-dayers.
The fifth and final match will be played at the same venue on Sunday.
Umar, 19, younger brother of the team's wicket-keeper Kamran Akmal, pounded four sixes and five boundaries in only his third international.
Younis stayed rock-solid at the other end as Umar punished the wayward Sri Lankan attack during a fifth-wicket partnership of 176 runs after Pakistan slipped to 130-4.
Sri Lanka appeared to make light of the daunting target when they moved to 101-1 in the 16th over, before losing their last nine wickets for 74 runs.
Left-handed opener Upul Tharanga top-scored with 80 off 89 balls and skipper Kumar Sangakkara made 39, but none of the other top order batsmen delivered.