Why did Buddha passed away in fourth Jhana? I just wonder why Buddha attain Parinibbana in fourth Jhana.
 Why not first or say second Jhana?

Maha-parinibbana Sutta: The Great Discourse on the Total Unbinding
Buddha emerged from the 4th jhana and passed away with equanimity; to demonstrate he passed away with an ordinary mind viewing ordinary reality (rather than passing away in an altered state of consciousness). This shows faithful disciples they do not need jhana or any non-ordinary mind to pass away at peace. All that is needed is equanimity &amp; wisdom. 


  Then the Blessed One, emerging from the cessation of perception &amp; feeling, entered the dimension of neither perception nor
  non-perception. Emerging from that, he entered the dimension of
  nothingness... the dimension of the infinitude of consciousness... the
  dimension of the infinitude of space... the fourth jhana... the
  third... the second... the first jhana. Emerging from the first jhana
  he entered the second... the third... the fourth jhana. Emerging from
  the fourth jhana, he immediately was totally Unbound. DN 16


However, the Buddha practised the jhanas before his passing to show he had not lost any mental mastery; per MN 12: 


  I am now old, aged, burdened with years, advanced in life, and come to the last stage: my years have turned eighty. Now suppose that I had
  four disciples with a hundred years' lifespan, perfect in mindfulness,
  retentiveness, memory and lucidity of wisdom. Just as a skilled
  archer, trained, practiced and tested, could easily shoot a light
  arrow across the shadow of a palm tree, suppose that they were even to
  that extent perfect in mindfulness, retentiveness, memory and lucidity
  of wisdom. Suppose that they continuously asked me about the four
  foundations of mindfulness and that I answered them when asked and
  that they remembered each answer of mine and never asked a subsidiary
  question or paused except to eat, drink, consume food, taste, urinate,
  defecate and rest in order to remove sleepiness and tiredness. Still
  the Tathagata's exposition of the Dhamma, his explanations of factors
  of the Dhamma, and his replies to questions would not yet come to an
  end, but meanwhile those four disciples of mine with their hundred
  years' lifespan would have died at the end of those hundred years.
  Sariputta, even if you have to carry me about on a bed, still there
  will be no change in the lucidity of the Tathagata's wisdom. MN 12