Warning: SPOILERS Below for Avengers: Infinity War!
Black Widow only played a supporting role in Avengers: Infinity War but Avengers 4 should remedy that and give her more prominence. After all, the super spy portrayed by Scarlett Johansson is the most experienced female superhero in the Marvel Cinematic Universe (with the most red on her ledger). The Widow is the only female founding member of the Avengers and she's been an integral part of the previous films. While she still awaits her own solo adventure, Black Widow deserves better in the next team up.
With much of Infinity War focused on Thanos' attempts to capture the Infinity Stones in outer space, Natasha Romanoff was one of the major characters whose involvement was limited to protecting the Mind Stone ensconced in the Vision's forehead from the Black Order in Wakanda. For the two years since turning her back on the Sokovia Accords in Captain America: Civil War, the Widow has been an international fugitive on the run with Steve Rogers. Judging from their ragged demeanor and terse dialogue - Natasha's lines in Infinity War amount to efficient quips like "We don't want to kill you, but we will", "Where's your friend?" and "She's not alone" - their time on the lam hasn't been easy.
Natasha, one of the few Avengers with no superpowers at all, is once more pulled back into a desperate war against a horde of aliens, serving as a field commander along with Rogers and Black Panther. She may be wary, but Romanoff shows no signs of intimidation. She bravely faces the Black Order and goes toe-to-toe with Proxima Midnight in battle, teaming up with fellow female heroes Scarlet Witch and Okoye in one of the film's crowd-pleasing fight scenes. Widow turns out to be one of the survivors of Thanos wiping out half of the universe with the Infinity Gauntlet. At the movie's ending, she's as bewildered as the others about what just happened.
Black Widow surviving Thanos's finger snap - which whittled down the number of characters left for Avengers 4 - hopefully means she'll get a larger role and renewed emphasis. There's a lot Natasha can offer the coming fight to try to reverse what the Mad Titan did to the universe, not the least of which is her battle-tested experience. It's hard to guess what moves in retaliation the Avengers will make in the next film, but Romanoff's unique abilities as an infiltrator and a spy should be placed to better use. Hawkeye didn't appear in Infinity War; assuming he didn't turn to dust, fans would enjoy a reunion between the former S.H.I.E.L.D. duo so they can bring the fight together - just like old times.
There's one other key narrative aspect involving Black Widow likely to be used in Avengers 4: her relationship with Hulk. The gamma-powered goliath refused to emerge after Thanos humiliated Hulk at the start of Infinity War. Since Avengers: Age of Ultron, Natasha and Banner have been in an on-again, (mostly) off-again love story that was only momentarily alluded to in Avengers 3. As we saw in Thor: Ragnarok, Banner and Hulk were off-world for two years on the planet Sakaar; the way he chose to turn his back on Natasha before he disappeared left her hurt and confused, though she buried her feelings and continued being an Avenger.
Now, the heroes don't have a Hulk but they need him more than ever. In Avengers 4, we could see a (likely bittersweet) resolution to the Natasha/Banner romance so that Natasha helps Banner unleash the Hulk once more. Since Banner desperately fears disappearing permanently if he reverts to being the Hulk again, this may result in Natasha saying goodbye to Bruce for the last time.
The obvious risk here would be repeating Age of Ultron and making Nat's entire role in the film center around this - otherwise, we're still restricting Black Widow. Avengers 4 must utilize all of her cunning, leadership skills, and deadly combat prowess as well, to truly do Black Widow justice. After all, the promised Black Widow movie should be a celebration of Johansson's character, not an attempt to save her.