Simplify the following text for a Grade 15 native English-speaking person.
Original: On going through his papers , Messiaen 's widow discovered that he had been composing a concerto for four musicians he felt particularly grateful to , namely Loriod , the cellist Mstislav Rostropovich , the oboist Heinz Holliger and the flautist Catherine Cantin ( hence the title Concert Ã quatre ) .
Simplified: Messiaen had also been composing a concerto for four musicians he felt particularly grateful to : his wife Loriod , the cellist Mstislav Rostropovich , the oboist Heinz Holliger and the flautist Catherine Cantin .