Anatoly Karpov gained the initiative yesterday in Moscow, defeating 22-year-old challenger Gary Kasparov in the fourth game of their world championship chess match. The score now stands at 2-2 in the match, which will be limited to 24 games. If the score is even at that point, Karpov will retain his title.

This game, begun Thursday in Moscow's Tchaikovsky Concert Hall and resumed yesterday after an overnight adjournment, was Karpov's first victory in his last 25 games against Kasparov and represents a significant improvement in his psychological position. The burden of playing to win has now been shifted back to the challenger.

Karpov resumed play yesterday with a series of repeated moves that hinted he was thinking of playing for a draw, which he could have had by repeating his 43rd and 44th moves. Then, by moving a bishop and pawn, he signified readiness to go for a win, and his play became more aggressive; he avoided a queen exchange that would have seriously reduced his winning chances.

Kasparov seemed not fully aware of Karpov's attacking possibilities, even after the champion had managed to get his rook back into active play; with a movement of his queen's pawn and later with a queen check from b7 that provoked a devastating response, he seemed to be still looking for a possible win when he should have been tightening his lines of defense.

In the final position, Karpov is threatening, among other things, 64. Qb8ch, leading to a quick mate. Kasparov must move either his queen to protect the back rank or his rook to make an escape hatch for his king. One possible winning sequence for Karpov might have gone: 63. . . . Re7; 64. Qf4ch, Ke8 (not Rf7; 65. Qb8ch, leading to mate); 65. Bg6ch, Kd8; 66. Qb8ch, Qc8; 67. Qxc8ch, Kxc8; 68. Rxe7, followed by an easy victory. Another possible line would be 63. . . . Rf6; 64. Qb8ch, Kf7; 65. Qg8, mate. Variations following 63. . . . Qd8 or . . . Qc8 are less elegant but no less lethal. WhiteBlack KarpovKasparov 1. d4d5 2. c4e6 3. Nc3Be7 4. Nf3Nf6 5. Bg5h6 6. Bxf6Bxf6 7. e30-0 8. Qc2Na6 9. Rd1c5 10. dxc5Qa5 11. cxd5Nxc5 12. Qd2Rd8 13. Nd4exd5 14. Be2Qb6 15. 0-0Ne4 16. Qc2Nxc3 17. Qxc3Be6 18. Qc2R(a)c8 19. Qb1Rc7 20. Rd2R(d)c8 21. Nxe6fxe6 22. Bg4Rc4 23. h3Qc6 24. Qd3Kh8 25. R(f)d1a5 26. B3Rc3 27. Qe2Rf8 28. Bh5b5 29. Bg6Bd8 30. Bd3b4 31. Qg4Qe8 32. e4Bg5 33. Rc2Rxc2 34. Bxc2Qc6 35. Qe2Qc5 36. Rf1Qc3 37. exd5exd5 38. Bb1Qd2 39. Qe5Rd8 40. f5Kg8 41. Qe6chKh8 42. Qg6Kg8 43. Qe6chKh8 44. Bf5Qc3 45. Qg6Kg8 46. Be6chKh8 47. Bf5Kg8 48. g3Kf8 49. Kg2Qf6 50. Qh7Qf7 51. h4Bd2 52. Rd1Bc3 53. Rd3Rd6 54. Rf3Ke7 55. Qh8d4 56. Qc8Rf6 57. Qc5chKe8 58. Rf4Qb7ch 59. Re4chKf7 60. Qc4chKf8 61. Bh7Rf7 62. Qe6Qd7 63. Qe5Resigns