West Indies lost wickets at regular intervals and with just one solid partnership, it managed to post 99 for 7 in 20 overs. A half-century to Bates led New Zealand to a comfortable victory, who finished at 103 for 2 in 16.3 overs.
Morna Nielsen made early inroads after West Indies opted to bat, dismissing Shaquana Quintyne and Kyshona Knight in successive deliveries in the fifth over. Shemaine Campbelle and Kycia Knight then got together to forge a 45-run stand for the fourth wicket, but Nicola Browne had Campbelle caught by Maddie Green for a 32-ball 29.
Kycia and Merissa Aguilleira (16) then put on 20 runs together for the fifth wicket, which was the last substantial partnership. West Indies managed to score only 13 runs after opener Kycia was dismissed for 28, before its quota of overs was complete. Nielsen and Bates picked up two wickets apiece.
Captain Bates and McGlashan came together to share a match-winning partnership of 75 runs in 12.2 overs for the second wicket, after opener Sam Curtis was run out in the second over. After Bates was dismissed for 50 by Anisa Mohammed, Sophie Devine and McGlashan saw New Zealand through without the loss of another wicket.
With Bates scoring her first half-century of the tournament, she shares the top spot for highest run-scorers in the series alongside Devine, with 101 runs at 33.66. She is also the leading wicket-taker in the series, picking up six wickets at an average of 6.66.
The latest win helped New Zealand seal the five-match series 3-0, with the third T20I washed out. The fifth and final game will be played on Sunday at the same venue.