In a match already reduced to 49 overs-per-side, and after Samuels blazed an unbeaten 106 (104 balls) in West Indies' total of 261 for 7, Pakistan was set a revised target of 189 off 31 overs as a second shower interrupted play when they were 68 for 2 off 17 overs.
Needing a further 121 off 14 overs, half-centuries from Mohammad Hafeez and Misbah-ul-Haq, the Pakistan captain, helped Pakistan overhaul the target with one over to spare.
Following a tie in the third ODI two days earlier, Pakistan now has an unbeatable 2-1 lead going into the fifth and final match at the same Beausejour Stadium venue on Wednesday.
The West Indies team seemed downcast when it took the field after being told by the umpires of the revised target, and Pakistan capitalised, Hafeez and Misbah hitting 54 runs off five overs before Dwayne Bravo, the West Indies captain, tried to get his team back into the match.
Hafeez fell to Kemar Roach for 59, an innings studded by five fours and two hooked sixes off Jason Holder, while Bravo himself accounted for Shahid Afridi who was promoted up the order to accelerate the scoring.
However, Umar Akmal (29 not out off 18) maintained the required scoring tempo, with Misbah speeding along to 53 off 43 balls at the other end to keep his team well on course. It was left to Akmal to finish off the match with three consecutive boundaries off Bravo.
Samuels stroked four sixes and nine fours in a feast of batting in the midday sunshine. He put on 95 for the fifth wicket with Lendl Simmons, who supported Samuels' classy assault with a belligerent 46 off 44 balls.