Solution 1: Harlequin
Have an extra player play NPC's. At start of scene, give them a card with their goals for the scene, and the NPC's sheet. If needed, give them also some information about what that character knows about the bigger picture. (But note that what they know may or may not be true.)
Solution 2: Expand the structure to 2+ Teams
Split the focus between team 1 and team 2, with players on each team. For example, if doing, say, any given police drama, each episode focuses on one team as the big, A-Story characters, with a smaller B-story by the other team. For Example, in Law and Order SVU, Bensen and Stabler are team A, and Tautuola and Munch are team B - in any given story, one of them is the lead story, getting about 2/3 of the episode, and the other team gets about 1/4 the episode. (minor overlap covers the remaining 1/12).
Solution 3: Narrow the Focus of the Characters
In Classic Trek, the "Trio" is Kirk, Spock and McCoy. Kirk is the action man and romance man, Spock the brains man and logic man, McCoy is the Heart Guy and medic. My last 4-player focused Bridge-Crew Trek game had the Captain who was Brains Man, psionics man and Social Sciences, the First Officer who was the Heart Guy and Action Man, the Doctor, who was both medic and logic guy, and the security chief and psionics guy. Note that the romance guy is absent, and the additional role still leaves some empty space for a 5th player. (Or that empty space can be used instead for some comedy of errors.)
Solution 4: Main or 2+ supports
Have players choose either to play 2-3 support characters, or one main character. Those who pick support characters get reduced points, and their two or three characters should be fairly different.
For example, if doing L&O Criminal Intent, Player 1 is Goran, Player 2 is Eams, Player 3 is the Captain and the Shrink, plus maybe Eams' boyfriend (in the show, not seen). A fourth player could be the Coroner and the ADA, and possibly Goran's mother.
TV in General: The Relationship Map.
The most important element in good TV emulation is a good clear map of the relationships.
Take, for example, Buffy. Who are the main characters? Well, for the series overall, Buffy, Giles and Xander. Why? Those three are in all seasons, and all other characters hang onto one or another of them. Monsters mostly attach to Buffy or Giles. In any given season, 1-3 others are 'main cast' - but the others vary by season. Anya, Tara, and Dawn are later seasons only. Willow goes away for most of a season, as does Giles. Giles doesn't get written out, tho - he guests every couple episodes - and his presence is still strongly felt even in his absence, and his prior relationships drive both a couple villains and the second tier characters. Willow's absence was fan-hurt, but not so much story loss, as most of her relationships overlap with Xander and/or Buffy.
Mapping the relationships out, however, reveals just how ensemble the plotting is for BTVS - Spike is linked to Angel, and Angel to both Xander (hostile) and Buffy (Romantic); Spike becomes linked to Giles, and later everyone. Anya starts linked to Cordelia, but then converts to Xander, and with Cordelia's absence, takes over Cordelia's role as Xander's love interest, and eventually Giles, as business partners.
Mapping out your party ahead of time allows them to come in with preestablished relationships, which makes the group dynamic much easier to maintain.
Round Robin with Troupe Style Rather than Group
Good start of game mapping can make it much easier to get a TV type feel. But also note that in TV-Land, not all the characters are in every scene. This can be emulated with what Marc Rein-Hagen dubbed "Troupe Style Play."
In Troupe Style play, every player has a main character, one that they alone play. The group also has a collection of secondary characters, which anyone not in the scene can play. These troupe-characters are included as needed, and usually, one or more are present in most scenes. They don't even have to be the same character to the same player. Focus on 1-2 of the main characters in the scene, and those not involved get to play a troupe character who could be in the scene.
Borrowing from Law & Order again, Dale Stucky could be a troup character, as could Dr. Wong, and any of the ADAs, plus a handful of recurrent uniforms, the Tech guy, and the Coroner. In such a game, Tautuola, Munch, Stabler and Bensen are all PC's. Only 1 or 2 scenes per "episode" would be all 4 PC's; all the others might be 2 pc's and 1-2 extras. Or 1 PC and 2-3 extras.