Here is Constantine star actor Matt Ryan explaining.
The Trench Coat is being dusted off. #Constantine #Arrow @CW_Arrow @amellywood looking forward to it. #thisfall pic.twitter.com/fROWx4RJ9o— Matt Ryan (@mattryanreal) August 11, 2015
Constantine is coming to the CW's Arrow, which for season 4 I have heard will be called "Green Arrow".
Here is another link, http://www.hollywoodreporter.com/live-feed/matt-ryan-constantine-arrow-814661
I am mostly avoiding spoilers, but this looks like it will be awesome and it is something that Stephen Amell, Oliver Queen himself, has wanted for a while now. Adding John to the mix, even for one episode, changes or rather broadens Arrow's world. We got super-science in season 2 and some magic in season 3. Looks like season 4 might embrace the wakiness that is the DC Universe's magical realms.
Of course I have to ask...When will Zatanna be on?