PORT ELIZABETH, South Africa (Reuters) - Opener Quinton de Kock passed the half-century mark for the eighth time in nine innings across all formats to lift South Africa to a comfortable six-wicket victory over Sri Lanka in the fourth One-Day International at St George’s Park on Wednesday.

The home side chased down their modest target of 190 in 32.5 overs after winning the toss and electing to field, securing a 4-0 lead in the five-match series with the final fixture to be played in Cape Town on Saturday.

De Kock followed up his century in the previous game with 51 from 57 balls as he blasted South Africa to a fast start in pursuit of the win.

Captain Faf du Plessis chipped in with 43 and JP Duminy showed form after his return from long-term injury with 31 not out.

Sri Lanka were bowled out for 189 in 39.2 overs, their innings rescued to some extent by an extraordinary 58-run final wicket stand between Isuru Udana (78 from 57 balls) and Kasun Rajitha (0).

The pair came together with the score on 131 as Udana powered his way to the highest List A score of his career with seven fours and four sixes before he was finally caught by Du Plessis off all-rounder Andile Phehlukwayo (2-21).

Fast bowler Anrich Nortje (3-57) was the pick of the home attack, though he suffered in the late onslaught from Udana.

South Africa may be concerned at their inability to close out the innings, a feature of their cricket in recent times, with the World Cup starting in a few months.

 Scorecard

Sri Lanka
 Avishka Fernando     c JP Duminy b Anrich Nortje                     29
 Upul Tharanga        c Tabraiz Shamsi b Dale Steyn                    4
 Oshada Fernando      c Reeza Hendricks b Lungi Ngidi                  0
 Kusal Mendis         c Anrich Nortje b Andile Phehlukwayo            21
 Priyamal Perera      c Quinton de Kock b Anrich Nortje                0
 Kamindu Mendis       c Quinton de Kock b Anrich Nortje                9
 Dhananjaya de Silva  c Anrich Nortje b JP Duminy                     22
 Thisara Perera       c&b Tabraiz Shamsi                              12
 Isuru Udana          c Faf du Plessis b Andile Phehlukwayo           78
 Lasith Malinga       Run Out Reeza Hendricks                          0
 Kasun Rajitha        Not Out                                          0
 Extras               0b 2lb 0nb 0pen 12w                             14
 Total                (39.2 overs)                           189 all out
Fall of Wickets : 1-13 Tharanga, 2-19 Fernando, 3-51 Fernando, 4-51 Perera, 5-69 Mendis, 6-71 Mendis, 7-97 Perera, 8-123 de Silva, 9-131 Malinga, 10-189 Tillakaratna

 Bowling              Ov  Md  Rn  Wk  Econ  Ex
 Dale Steyn            7   0  32   1  4.57  1w
 Lungi Ngidi           5   1  22   1  4.40
 Anrich Nortje         8   0  57   3  7.12  2w
 Andile Phehlukwayo  5.2   0  21   2  3.94  2w
 Tabraiz Shamsi       10   0  34   1  3.40  3w
 JP Duminy             4   0  21   1  5.25

 South Africa 
 Quinton de Kock  c Upul Tharanga b Dhananjaya de Silva       51
 Reeza Hendricks  c Thisara Perera b Dhananjaya de Silva       8
 Aiden Markram    c Dhananjaya de Silva b Kasun Rajitha       29
 Faf du Plessis   c Priyamal Perera b Dhananjaya de Silva     43
 David Miller     Not Out                                     25
 JP Duminy        Not Out                                     31
 Extras           0b 1lb 0nb 0pen 2w                           3
 Total            (32.5 overs)                             190-4
Fall of Wickets : 1-21 Hendricks, 2-77 Markram, 3-110 de Kock, 4-144 du Plessis
Did Not Bat : Phehlukwayo, Steyn, Nortje, Ngidi, Shamsi

 Bowling               Ov  Md  Rn  Wk  Econ  Ex
 Dhananjaya de Silva   10   1  41   3  4.10
 Lasith Malinga         7   0  38   0  5.43  1w
 Isuru Udana            2   0  15   0  7.50
 Kasun Rajitha          5   0  32   1  6.40  1w
 Kamindu Mendis         4   0  34   0  8.50
 Oshada Fernando        2   0  16   0  8.00
 Thisara Perera       2.5   0  13   0  4.59