Tom Cavanagh has been a part of the main cast of The Flash for three years, while playing a different version of the same character each season. Fans of The CW series can expect that trend to continue into the show's fourth season, with the introduction of a new Harrison Wells.
At San Diego Comic-Con 2017, Cavanagh talked about the experience of playing multiple versions of Harrison Wells and also answered questions about what character he will play in The Flash season 4. Cavanagh says that the characters he played in the last two seasons were created to fit in with the story and the tone of the show. Harry, a "a jackass who has no social skills," appeared in season 2 to affect the "winning" dynamic of main characters such as Barry, Cisco, Caitlin, and Joe. In season 3, we were introduced to H.R., who brought humor to a dark story.
Cavanagh had this to say about what direction the show will take with Wells during The Flash season 4 (per CBR):
And so this year, we’ve had a different villain and so I think there’s going to be areas that we could use a different Wells. We’re going to start with Harry and then find a way, I think, to try and get a different version of Wells in there, be it some version of Harry or whatnot as the season’s gaps present themselves.
Cavanagh was careful not to go into detail about what that new version of Wells could be or how he would interact with the other characters. Other cast members claim to have no knowledge of who Cavanagh will be playing next. Since The Flash season 4 had already begun filming when the remarks were made, this suggests that the new Wells won't immediately appear and may be saved for later episodes in the season.
However, both Cavanagh and Carlos Valdes, who plays Cisco, confirmed that Harry will return in some capacity. Of the three major characters played by Cavanagh on the show, only Harry has managed to survive a single season. Though Harry was off-world for several episodes, the character did return sporadically and even had a role in the final battle with Savitar in the season 3 finale. When we last saw Harry, Barry asked him to stay on Earth-1 to look after the team in Barry's absence, which indicated that Harry would stay on as a member of Team Flash for the time being.
Season 4 of The Flash premierse on The CW at 8 PM on October 10.