We know there are 4 living prophets. I only know Isa is in the heavens and Khizr on the land. The two remaining, Idris and Ilyas, where are they? Any evidence from Quran or ahadith on these 4 living prophets? Will all 4 re-appear when 12th Imam (a) reappears?

Re Prophet Idris: check surah Mariam verse 57
Re Prophet Ilyas: historic narrations such as in Tabari point to Allah raising him to heavens

Yes they will all return with the holy 12th Imam, the awaited saviour, Inshallah