I just wonder why Buddha attain Parinibbana in fourth Jhana. Why not first or say second Jhana?
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 & wisdom.
Then the Blessed One, emerging from the cessation of perception & 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