Opening batsman De Kock (112) made his first international century to anchor the innings as South Africa posted 266 for five from their 50 overs after winning the toss in Abu Dhabi.
Pakistan were well placed to level the series at 2-2 when they were 3-174 with over 13 overs remaining, but Steyn took five for 25 as they collapsed to 238 all out.
It was paceman Steyn's best bowling figures in ODIs and the victory at Sheikh Zayed Stadium gave AB de Villiers' side an unassailable 3-1 lead with one game to play.
De Kock had some fortune when he was dropped on two, and Pakistan missed a run-out opportunity when he was five runs short of his century.
But the 20-year-old made the most of his good fortune by displaying some fine strokeplay before he was caught by Misbah-ul-Haq off Junaid Khan (2-42) in the 43rd over.
De Kock had shared an 87-run opening partnership with Hashim Amla before Mohammad Hafeez (2-34) bowled Amla for 46.
De Villiers chipped in with 30 before JP Duminy (25 not out) and Ryan McLaren (28 not out) put together a 50 partnership to ensure South Africa posted the highest score of the series.
Pakistan have not chased down over 250 since March 2011 but they made a solid start until three wickets in the space of 14 balls reduced them to 3-86.
Ahmed Shehzad (43), Hafeez (33) and Asad Shafiq (one) all departed in quick succession, but a fourth-wicket partnership of 88 between debutant Sohaib Maqsood (56) and Misbah (65) steadied the ship.
However, Magsood was on his way when de Villiers took a fine catch back-pedalling at mid-off to give Steyn his first wicket and a sharp bouncer from the quick in his next over accounted for Sohail Tanvir (one), caught by Amla at fine leg.
That left Pakistan on 5-180 needing a further 87 to win off 68 balls and they struggled to keep up with the run-rate before their hopes were dashed when Steyn dismissed Umar Akmal (22), Misbah and Saaed Ajmal (nought) in the 47th over to complete his five-wicket haul.
Shahid Afridi was run out for just three and then McLaren sealed the victory when he bowled Junaid (four).