THE MAN SAID IT HIMSELF. "The trench coat is being dusted off."
The rumors were true Constantine is coming on Arrow Season 4 for a "one time deal". Matt Ryan will be reprising his role on the 5th episode titled "Haunted" who will play a big role in the Resurrection of Sarah Lance.
...the prevous black canary who will come back as the white canary on [DC's Legends of Tomorrow](tag:2937021).
Constantine will be "Resurrected" back for the purpose of the Arrow Story and that is the main focus for now said, Arrow executive producer Wendy Mericle. as long as he is making an appearance i'm happy and the show fans are happy or at least that is how i feel i think with an appearance on arrow things might get better for Constantine he might be resurrected for a whole new season on CW or another TV channel/network.
personally it think it's a great move by Arrow team to introduce the magical and mystyical side of the DCU which might help character such as Zatanna, Etrigan the demon, The spectre, deadman, and many more to have their debut on Arrow, flash, supergirl or any other future DC show property.