New Zealand 315 for 8 (Williamson 97, Taylor 96) beat Sri Lanka 195 (Sangakkara 81, Anderson 4-52) by 120 runs
Scorecard and ball-by-ball details
A 117-run third-wicket stand between Kane Williamson and Ross Taylor was the centrepiece of New Zealand's imposing 315 for 8, which founded a 120-run victory against an ailing Sri Lanka. Williamson and Taylor hit 97 and 96 respectively, on a track not offering much beyond the first 20 overs, before Corey Anderson and Grant Elliott provided late-order acceleration to transform the total from dominant to daunting. Anderson also claimed 4 for 52 with the ball, as the hosts sewed up the series. They lead 4-1 with one match to play.
Sri Lanka arrived in New Zealand with high hopes for their southern-hemisphere season, but with one win out of seven matches in all, their World Cup plans have veered towards disarray. Once more, a vital catch was dropped. A straightforward chance from Williamson was spurned, and the batsman went on to make 70 more runs. Again the bowling failed to penetrate beyond the early overs, as the hosts' top order worked themselves into form.
And for the fourth time in the series, Sri Lanka's batsmen collapsed around a good innings from one of the senior three batsmen. It was Kumar Sangakkara's turn to wage a lone battle this time. He hit 81 from 66, as the team slipped to 195 all out in the 41st over.
In the first innings, Williamson played and missed while the ball was still new enough to move in the air and off the seam, but quickly established a tempo while Martin Guptill struggled at the other end. A back-foot punch off Suranga Lakmal brought him his first boundary, before an on-drive and a leg glance off Dhammika Prasad brought two more, a few overs later.
When Rangana Herath's spin arrived, Williamson used his feet adeptly, often trekking out of his ground to work balls into gaps. Herath should have had him for 27 in the 15th over, but Sri Lanka's trend of shelling vital catches continued. Dilshan got his hands to lofted cover-drive, but ended up parrying it over his head, having misjudged the pace at which the ball was coming to him.
Taylor was watchful early in his innings, as he sought to shake off a run of poor form. But he appeared to regain fluency after he hit Thisara Perera for four through the covers in the 18th over, and he and Williamson settled into a pattern of risk-free accumulation. They traveled at around four runs an over until Williamson raised the rate with two beautifully timed sixes over wide long on in the 24th and 26th overs, completing his half-century, off 60 balls, in between.
The pair continued smoothly as Lahiru Thirimanne got overs out of his less capable bowlers, partly to make up for those the still-injured Angelo Mathews would ordinarily contribute, but also because Lakmal had left the field with a groin strain, and could bowl no more than seven overs. Williamson was well on track to hitting his second hundred in three innings when he was run out, effectively by Taylor who did not respond to Williamson's call for a second. Williamson departed for 97 from 95 balls, despondent though he had put New Zealand on course for a second consecutive score of over 300.
Taylor completed his first fifty of the series in the same over Williamson fell, and quickly meted out punishment during the batting Powerplay, cracking Prasad square on the off side twice in the 37th over. A promoted Anderson began his charge too, walloping three fours himself in a Perera over that disappeared for 19, but just as he was getting his big hits humming, Herath had him caught at the cow corner fence.
Taylor would depart in sight of his 12th ODI ton not long after, trapped plumb in front by a yorker-length Dilshan ball. But though wickets tumbled towards the end of the innings, Grant Elliott's big strikes ensured that the foundation New Zealand had constructed at 40 overs - when they had 229 for 3 - would result in another commanding score.
That New Zealand did not achieve a total as great as their 360 on Friday, despite having been 175 for 2 after 34 overs, was thanks in part to Herath's control and Dhammika Prasad's intensity. The pair were brought into the side for this match in place of Nuwan Kulasekara and Sachithra Senanayake.
Sri Lanka's response began with some promise, as the openers progressed to 56 without loss in the 13th over, before a wild Dilshan swipe sent a nick through to the keeper, off Anderson. Thirimanne was strangled down the leg side next over for 29, as the team's efforts to find a decent opening partner for Dilshan had him batting at the top of the innings again.
Mahela Jayawardene was visibly out of touch, and was almost out twice to Daniel Vettori, who was playing a record 280th ODI for New Zealand. Though it was the spinner that troubled him, Jayawardene would be out to Anderson, scooping him gently to mid on in an attempt to correct his languishing scoring rate.
Kyle Mills, Trent Boult and Vettori all contributed mean spells of bowling, sending the asking rate ever higher, and it was that pressure that saw Dimuth Karunaratne hole out for 26, and saw both Dinesh Chandimal and Jeevan Mendis cheaply run out. Sri Lanka were always going to struggle after a big top-order partnership eluded them, but Sangakkara's dismissal in the 38th over effectively sealed the loss. He was out trying to lift his tour nemesis Boult over the inner ring. A third run-out and another high catch in the circle finished the match off.

Andrew Fidel Fernando is ESPNcricinfo's Sri Lanka correspondent. @andrewffernando