Felicity Smoak, played by Emily Bett Rickards, is a going to once again going to be crossing paths with Barry Allen.
Felicity will be appearing on an early episode of The Flash during the upcoming forth season, prior to the yearly big crossover between the CW shows. The episode will be titled "Girl's Night Out", so you can draw your own conclusions from that. It also will not be the first time she has appeared in her crossover episode with The Flash, after also appearing in the first season in "Going Rogue" and the third season episode "Paradox".
Katee Sackhoff will also appear in the episode as the villain Blacksmith. She is the head of an underground black market for supervillains that she uses to make sure she stays successful.