Olivier Eugene Charles Prosper Messiaen (French: [olivje mesja]; December 10, 1908 - April 27, 1992) was a French composer, organist, and ornithologist, one of the major composers of the 20th century. His music is rhythmically complex; harmonically and melodically he employs a system he called modes of limited transposition, which he abstracted from the systems of material generated by his early compositions and improvisations. He wrote music for chamber ensembles and orchestra, vocal music, as well as for solo organ and piano, and also experimented with the use of novel electronic instruments developed in Europe during his lifetime. He travelled widely and wrote works inspired by diverse influences ranging from Japanese music, the landscape of Bryce Canyon in Utah and the life of St. Francis of Assisi.

Messiaen's next work was the large-scale La Transfiguration de Notre Seigneur Jesus-Christ. The composition occupied him from 1965 to 1969 and the musicians employed include a 100-voice ten-part choir, seven solo instruments and large orchestra. Its fourteen movements are a meditation on the story of Christ's Transfiguration. Shortly after its completion, Messiaen received a commission from Alice Tully for a work to celebrate the U.S. bicentennial. He arranged a visit to the US in spring 1972, and was inspired by Bryce Canyon in Utah, where he observed the canyon's distinctive colours and birdsong. The twelve-movement orchestral piece Des canyons aux etoiles... was the result, first performed in 1974 in New York.  In 1971, he was asked to compose a piece for the Paris Opera. While reluctant to undertake such a major project, he was persuaded in 1975 to accept the commission and began work on his Saint-Francois d'Assise. The composition was intensive (he also wrote his own libretto) and occupied him from 1975 to 1979; the orchestration was carried out from 1979 until 1983. Messiaen preferred to describe the final work as a "spectacle" rather than an opera. It was first performed in 1983. Some commentators at the time thought that the opera would be his valediction (at times Messiaen himself believed so), but he continued to compose. In 1984 he published a major collection of organ pieces, Livre du Saint Sacrement; other works include birdsong pieces for solo piano, and works for piano with orchestra.  In the summer of 1978, Messiaen retired from teaching at the Conservatoire. He was promoted to the highest rank of the Legion d'honneur, the Grand-Croix, in 1987. An operation prevented his participation in the celebration of his 70th birthday in 1978, but in 1988 tributes for Messiaen's 80th included a complete performance in London's Royal Festival Hall of St. Francois, which the composer attended, and Erato's publication of a seventeen-CD collection of Messiaen's music including a disc of the composer in conversation with Claude Samuel.  Although in considerable pain near the end of his life (requiring repeated surgery on his back) he was able to fulfil a commission from the New York Philharmonic Orchestra, Eclairs sur l'au-dela..., which was premiered six months after his death. He died in Paris on April 27, 1992.  On going through his papers, Loriod discovered that, in the last months of his life, he had been composing a concerto for four musicians he felt particularly grateful to, namely herself, the cellist Mstislav Rostropovich, the oboist Heinz Holliger and the flautist Catherine Cantin (hence the title Concert a quatre). Four of the five intended movements were substantially complete; Yvonne Loriod undertook the orchestration of the second half of the first movement and of the whole of the fourth with advice from George Benjamin. It was premiered by the dedicatees in September 1994.

Using a quote from the above article, answer the following question: what was significant about that work?
The composition occupied him from 1965 to 1969 and the musicians employed include a 100-voice ten-part choir, seven solo instruments and large orchestra.