According to Entertainment Weekly who got the exclusive Echo Kellum (Ben and Kate) has been cast as Mr. Terrific who’s been confirmed for season 4 of Arrow.
Curtis Holt who is Mr. Terrific, is said to be a technological savant and inventor who works at Palmer Technologies under the supervision of Felicity Smoak for season 4 of The CW television series.
In the comics, Mr. Terrific’s alter-ego is Michael Holt, an extremely intelligent Olympic-level athlete who joined the justice Society of America following the death of his wife and unborn child.