In the poem of Walt Whitman, [I celebrate myself, and sing myself], what do the last four lines mean? Thank you very much for your answer.
I believe you are referring the concluding lines of section one and not the entire poem (52 sections long) so I will comment on that aspect.
Those lines read:
Creeds and schools in abeyance,
Retiring back awhile sufficed at what they are, but never forgotten,
I harbor for good or bad, I permit to speak at every hazard,
Nature without check with original energy.
In the first line, Whitman is retiring dogma. "Creeds and schools," that is, the segmenting of thought and discipline, have been suspended for a time.
The second line says that he has not forgotten the lessons learned, but for now, they are only that...lessons, not living itself.
The third line recognizes that the speaker is going to speak his mind no matter what the personal cost. He will reveal in the sections that follow both the good and the bad about himself, come what may.
The fourth line sums up the whole section: "Nature without check and with original energy" realizes that nature without "check" can be wild, untamed. (Think of a mowed lawn vs. a meadow). The "original energy" is that in our nature that existed before the manicuring of our true, uninhibited selves.
Part 1 of Whitman's "Song of Myself" ("I celebrate myself, and I sing myself") is divided into four irregular stanzas, and concludes with a four-line stanza which, effectively, summarizes the whole. This section of Whitman's longer poem is about unity between people, animals, "summer grass" and everything on earth which shares the same atoms—which is to say, everything that is alive.
Rendered into straightforward prose, then, the last four lines of the section mean: Structured systems of thought and schools are in a state of suspension. Thinking back, I recognize I have not forgotten the lessons I learned in school, but they did not capture any more than they had to. Now, I am a safe harbor in what I say, whether people respond to it well or badly; I will speak openly about everything, whatever the cost. I will speak about nature—and will speak of my own nature—freely, and using the energy bestowed upon me by nature, rather than any of the lessons that have been taught to me by structured schooling.