Mohammad Hafeez contributed with 70 as the travelers surpassed Sri Lanka's 256-9 in the 41st over at the Premadasa stadium to take a definitive 3-1 lead in the arrangement.
The arrangement win kept Azhar Ali's Pakistan in conflict for a spot in the eight-country Champions Trophy in Britain in 2017.
Pakistan are two focuses in front of the West Indies at number eight in the rankings, yet both groups are because of play a tri-arrangement in Zimbabwe one month from now that will focus the last spot.
Pakistan's one-day achievement takes after their 2-1 triumph over Sri Lanka in the former Test arrangement under Misbah-ul Haq's captaincy.
Sri Lanka are guaranteed of participating in the Champions Trophy, yet their dull showcase did not do equity to the previous title holders.
Shehzad and Azhar put Pakistan making progress toward triumph with an opening stand of 75 in 11 overs before the captain was uprooted by sling-arm quick bowler Lasith Malinga for 33.
Hafeez helped Shehzad expand on the fine begin by including 115 for the second wicket, however Sri Lanka hit back to uproot both batsmen in the space of 23 runs.
Shehzad fell five runs shy of his fourth one-day century when he slice seamer Suranga Lakmal to a jumping Kusal Perera at third man. He hit 12 limits and a six.
Hafeez was gotten behind by Dinesh Chandimal off spinner Milinda Siriwardana to leave Pakistan on 213-3, however Sri Lanka's recuperation came past the point of no return.
Shoaib Malik hit a 16-ball 29 and finished the match by hurling Siriwardana over mid-wicket for his fourth six. Sarfraz Ahmed stayed unbeaten on 17.
Prior, Lahiru Thirimanne top-scored with 90 for Sri Lanka after captain Angelo Mathews won the hurl interestingly on Pakistan's present visit and chose to bat.
The left-hander put on 109 for the second wicket with Tillakaratne Dilshan (50) after Perera had been released by Mohammad Irfan off the second bundle of the match.
Thirimanne missed his fifth one-day century when he cleared leg-spinner Yasir Shah to profound square-leg, where Shehzad got a low catch to present appropriate reparations in light of a dropped catch in the bowler's past over.
Other than Thirimanne and Dilshan, none of the other top-request batsmen settled in as Mathews succumbed to 12 and Dinesh Chandimal was released subsequent to making 20.
Tailender Malinga crushed 17 off 13 balls toward the end to guarantee Sri Lanka crossed the 250-run mark.
Irfan completed with three wickets, while individual seamer Anwar Ali took two.
The fifth match will be played in Hambantota on Sunday.