At one point, I considered picking up the Glek variation, but I found the variation of 4... d5 to be far too problematic. Keep in mind, that the principle idea of the variation is to retain a knowledge advantage, but most importantly, retain plenty of opportunity for out-playing the opponent.
On this line, the d5 line puts direct challenge to the idea. Further, 5. exd5 Nxd5 6. Bg2 Nxc3 7. bxc3 From here, you should find a nice niche line. I recommend starting with 7... Bc5 or Bd6.
If you put any length of study into this line, you will face no issue what so ever in this variation. Most important though, especially for this line, is to master the principles of the strategy here instead of long series of lines.
Sample Game
Here is a nice execution by the French GM Laurent Fressinet where he managed to win against this opening (played against the eponym himself, the German GM Igor Glek)
[FEN ""]
[Site "Katernberg GER"]
[Round "13"]
[White "Glek, Igor"]
[Black "Fressinet, Laurent"]
[WhiteElo "2537"]
[BlackElo "2658"]
[Result "0-1"]
[ECO "C47"]
[Opening "Four Knights"]
[PlyCount "111"]
1. e4 e5 2. Nf3 Nc6 3. Nc3 Nf6 4. g3 d5 5. exd5 Nxd5 6. Bg2 Nxc3 7. bxc3
Bc5 8. O-O O-O 9. d3 Bg4 10. h3 Bh5 11. g4 Bg6 12. Ng5 h6 13. Ne4 Bb6 14.
Ba3 Re8 15. Kh2 Qd7 16. Rb1 Nd8 17. f4 exf4 18. Rxf4 Ne6 19. Rf1 c6 20. Kh1
Bxe4 21. Bxe4 Ng5 22. Bg2 Re6 23. Rf5 g6 24. Rf4 Rae8 25. Qf1 Bc7 26. Rf6
Bb8 27. Bc5 Re2 28. Bg1 Rxc2 29. Rf2 Rxc3 30. Re2 Rxe2 31. Qxe2 Rxd3 32. h4
Ne6 33. Rxb7 Qxb7 34. Qxd3 Qc7 35. Qb3 c5 36. Bd5 Qf4 37. a4 Be5 38. Qf3
Qxf3+ 39. Bxf3 Bd4 40. Bh2 c4 41. Kg2 c3 42. Bd1 Bf6 43. Bg1 Nd4 44. Bxd4
Bxd4 45. Kf3 Kg7 46. Kf4 Bf2 47. h5 Kf6 48. Kf3 Bb6 49. Ke4 Kg5 50. Kd3 Ba5
51. Be2 gxh5 52. gxh5 f5 53. Bd1 Kf4 54. Ke2 Kg3 55. Kf1 f4 56. Bc2 0-1