So I counted all of the performance's since Rumour Has It/Someone Like You (which was the 300th), including unreleased ones. Here is the list:

1. Perfect

2. I'm The Only One

3. Girls Just Wanna Have Fun

4. Jolene

5. I Kissed A Girl

6. Constant Craving

7. Red Solo Cup

8. Buenos Aires

9. Survivor/I Will Survive

10. ABC

11. Control

12. Man In The Mirror

13. We Are Young

14. All I Want For Christmas Is You

15. Blue Christmas

16. River

17. Extraordinary Merry Christmas

18. Let It Snow

19. My Favourite Things

20. Santa Claus Is Coming To Town

21. Christmas Wrapping

22. Do They Know It's Christmas? (Feed The World)

23. Summer Nights

24. Wedding Bell Blues

25. Moves Like Jagger/Jumping Jack Flash

26. The First Time Ever I Saw Your Face

27. Without You

28. We Found Love

29. Wanna Be Startin' Something

30. Bad

31. Scream

32. Never Can Say Goodbye

33. Human Nature

34. Ben

35. Smooth Criminal

36. I Just Can't Stop Loving You

37. Black Or White

38. La Cucaracha

39. Sexy And I Know It

40. Dang Diggy Dang

41. Don't Wanna Lose You

42. Bamboleo/Hero

43. La Isla Bonita

44. A Little Less Conversation

45. Chapel Of Love

46. L-O-V-E

47. Let Me Love You

48. Stereo Hearts

49. Home

50. I Will Always Love You

51. You're The Top

52. Cherish/Cherish

53. Love Shack

54. Cough Syrup

55. Stand

56. Glad You Came

57. She Walks In Beauty

58. Fly/I Believe I Can Fly

59. What Doesn't Kill You (Stronger)

60. Here's To Us

61. I'm Still Standing

62. Hungry Like The Wolf/Rio

63. Fighter

64. Up Up Up

65. Somebody That I Used To Know

66. You Should Be Dancing

67. Night Fever

68. Disco Inferno

69. If I Can't Have You

70. How Deep Is Your Love?

71. Boogie Shoes

72. More Than A Woman

73. Stayin' Alive

74. How Will I Know

75. I Wanna Dance With Somebody (Who Loves Me)

76. Saving All My Love For You

77. So Emotional

78. It's Not Right But It's Okay

79. I Have Nothing

80. My Love Is Your Love

81. The Music Of The Night

82. School's Out

83. Cell Block Tango

84. Not The Boy Next Door

85. Don't Rain On My Parade

86. The Rain In Spain

87. Shake It Out

88. Cry

89. Big Girls Don't Cry

90. Dinosaur

91. Love You Like A Love Song

92. What Makes You Beautiful

93. Take My Breath Away

94. I Won't Give Up

95. Because You Loved Me

96. Mean

97. Flashdance... What A Feeling

98. Edge Of Glory

99. It's All Coming Back To Me Know

100. PARADISE BY THE DASHBOARD LIGHT!

So The 400th perrformance is Dashboard and seeing as it was one of their best I think it's fitting. Without the unreleased performances they still have not reached the 400th performance - with We Are The Champions being 399th. That means the first performance from Goodbye will be the 400th released performance :D xo