The 33-year-old right-hander Hafeez followed up his tons in the first and third one-day internationals with another masterful performance in Abu Dhabi on Wednesday as an unbeaten 113 led his side to overall victory with one match to spare.
Hafeez has been composed at the crease throughout the series, and he once again showed the full array of shots in his armoury as he built an innings that included 12 fours and two sixes in 119 balls to once again claim the headlines.
He was ably supported by Ahmed Shehzad (44) and then Sohaib Maqsood (46 not out), with an unbeaten 111-run third-wicket stand with the latter guiding the hosts home after Sharjeel Khan had gone early in their chase.
He was deceived on 13 playing the wrong line to a slower ball from Angelo Matthews, which beat the edge on the way to hitting off stump, as Pakistan faltered.
But from there, they were always in control as they chased down an achievable 226 target - the result largely of Gul's earlier efforts with the ball - with almost nine overs to spare.
The 29-year-old paceman Gul had returned on Sunday from a nine-month injury lay-off to lead Pakistan to a one-sided 113-run victory in the third match of the series.
And he was at it again here, ripping through the Sri Lanka top order with match figures of 3-37 off seven overs to reduce the tourists at one stage to 36-3.
There was some solace for Sri Lanka courtesy of a brief recovery led by a half-century from Kumar Sangakkara (51) and debutant Ashan Priyanjan, whose knock of 74 was ended when he was bowled by Junaid Khan (2-42 off nine) but still bodes well for a fruitful career at this level for the former Sri Lanka Under-19 captain.
Sangakkara's run out and Priyanjan's eventual exit left the tourists 165-5 on a docile pitch, but they were soon derailed again when Nuwan Kulasekara (one) fell to Saeed Ajmal (4-39 off 9.5) just nine balls later.
Matthews (38) and Kithuruwan Vithanage (27) did at least push the total on, but it was never likely to be enough once Hafeez hit his stride.
The fifth and final match of the series takes place on Friday.