EW writes that Tom Felton has yet to sign on for any episodes of Season 4 of The Flash, which would explain why he was not present in the trailer or at San Diego Comic Con for The Flash panel.
His character, Julian Albert, was still a member of Team Flash as the show closed out Season 3, so it’s assumed that his departure happened in the six months that followed and that someone in the show will acknowledge his leaving.
Having said that, the news being reported is that Felton has not signed back on as a series regular, which means that there is still a chance he could return to the series in a guest role either later in Season 4, or down the road in the series.