Probably this year. TNK usually releases 1 anime / year. Each DxD season was their only release in that year. They haven't released any anime yet this year. Between season 1 and 2 there were 5 "anime seasons" (spring, summer, fall, winter, spring)
Season 3 was released last year spring season, this means that so far 4 seasons went by. (summer, fall, winter, spring) If they go back to their pacing between S1 and 2 then S4 should be released during fall but I suppose they might surprise us and release it during the summer season although time's running out for an announcement.