The Marvel Cinematic Universe is largely celebrated for its interconnected worlds, crossing over a vast amount of characters with each other. Fans saw the pinnacle of this in Avengers: Infinity War, which contained the largest number of main characters in any Marvel movie. Infinity War famously wiped the board clean in its epic ending, leaving plenty more room for other characters to share the spotlight. However, that doesn't mean that every major character is going to be in Avengers 4. Actually, there's a good chance that we see won't see certain characters ever again.
Not everybody gets to be as popular as Iron Man, Captain America, Black Widow, or Spider-Man. Plenty of characters have graced the big screen flying under the Marvel Studios banner and not all of them catch on with audiences. The reasons tend to vary but even Marvel has its misfires, and it's highly unlikely that any of these still alive, major, sides characters appear in Avengers 4.
One of the rare villains to make it out of his debut movie alive, Abomination might as well be good as dead. The monstrous villain hasn't been seen or heard of since The Incredible Hulk, which is pretty understandable. Given how the whole Edward Norton thing shook out, Marvel really wants to forget this movie exists. While there are certainly plenty of ways Abomination could be utilized in the future, this guy hasn't seen the light of day since 2008, so don't count on him showing up. Then again, it wouldn't be the first time that Marvel brought back an almost forgotten villain.
Thor: Ragnarok essentially ditched most of Thor's supporting cast in favor of a fresh start, and that included Jane Foster (Natalie Portman). Jane was never really able to escape the fact that she was a love interest, with much of her screen time spent pining for Thor. This got especially bad in Thor: The Dark World where she was a damsel in distress for over half of the movie. There were also reports that Portman wasn't too pleased with the production of the movie, meaning she was probably okay that she never got a callback. That being said, Marvel has changed a lot since those early days, and Portman has expressed a willingness to return if she were ever asked.