This game is not immune to criticism despite my respect for the series.
The review begins with technical aspects.

The art style and animation is smoother than trailers made them look. Streets Of Rage 4 looks and feels wonderful. Attention to detail is top notch.
The music fits the environments and theme of the game extremely well, enhancing the mood and experience overall. Nothing aesthetic felt out of place.

If you're an achievement hunter this game won't disappoint. The toughest ones are not absurd but test your mastery of the game. This is what achievements should be.

Many people complain about characters moving slowly and I don't see why.
The pacing matches Streets Of Rage 2.
What I found confusing was the lack of a running command. In Streets Of Rage 3 all characters are able to run, and roll vertically, by double tapping a direction. All Streets Of Rage 3 characters available in Streets Of Rage 4 retain this functionality, but no main characters from Streets Of Rage 4 have these abilities.
This feels like a step backwards.

You can't block either. After this many years you'd think the ability to block a punch would be added, even if in a somewhat limited form.
Why do I have to avoid hits by attacking first?
Maybe they could not implement this without making the game too easy, but this problem is mitigated by the counterattack. Counterattacking fits the game's non-stop action format and tests your spatial awareness.

New enemies blend well with old ones and add considerations when entering an area. Old enemies are not boring or predictable to the point of reducing challenge. You also get to fight police and riot police. I loved seeing thugs and police attack each other while I went to get an item. Never before have I seen this kind of thing in a beat em' up. Environmental hazards proved fun and not overwhelming. I loved making enemies walk through toxic waste as I fought someone else. Being kicked into a pit doesn't instantly kill you, but it's damaging enough to make you be careful. Boss fights were fun and the difficulty between them was evenly dispersed throughout the game. The bartender from Streets Of Rage 2 returns, and once again you fight him in the alley behind the bar. From fighting the police chief in his office to the alcoholic dominatrix on a boat, there is always a fun surprise at the end of a level. Every level has a fun mix of old and new weapons.

Some enemies have annoying one frame attacks you can't avoid, which is the pinnacle of artificial difficulty. Having a high combo count broken by the game deciding you can't succeed is not fun.
Juggling enemies and bouncing them off objects is wonderful, and chaining combos into special attacks makes crowded places lots of fun. I also like the ability to regain the energy lost from using these skills by attacking enemies, possibly losing it if you're hit beforehand. This gives a sense of risk and reward I didn't experience in the older games.

Unlocking new characters as you played the game was fun, and I had a great time experimenting between all the new and old characters. The old ones play as well as ever and I'm inclined to say Blaze from from the first two games is overpowered. She seemed unusually strong and I can't tell if this was intentional.
I'm disappointed they excluded Roo. Seeing him behind the bar was misleading and disappointing.

In the end nothing can be perfect, but this game delivers.
I consider it the best in the series.

Now I'll talk about why Streets Of Rage has often been regarded as the royalty of its genre.

As a child in the 90s you were indirectly taught to idolize and revere vigilante justice.
Our cartoons like the Ninja Turtles, Bucky O'Hare and T-Rex, told us it was cool to be a secret group of friends fighting crime and oppression. Our superheroes like Batman and Spiderman, were portrayed as acceptable exceptions to people taking the law into their own hands.

Our video games were hardly different.
The infamous Double Dragon may not have been the first beat em' up, but it set the standard for the genre. Unfortunately it suffered from incoherent settings and poor continuity. Whether it was street gangs in the Gameboy version of Double Dragon 2, or Double Dragon 4 taking place in a post apocalyptic wasteland vaguely reminiscent of Fist Of The North Star, the series lacks a consistent theme outside of its mechanics.

Final Fight is the other big contender for these classic games. With a main character being the police chief on a rampage, these games hold true to the theme of vigilante justice. But the stories play out like cheesy action movies.

Some other games, like Rival Turf, started in the right place but quickly ended up somewhat generic. It suffered from poor quality, low budget, and boss fights not fitting the narrative.

Streets Of Rage dared to be dark and realistic about the idea of vigilante justice.
When Double Dragon 3 gave us a mystical martial arts prophecy about magic stones, Streets Of Rage gave us ex-police officers fighting an organized crime syndicate.

Final Fight gave us sappy love and rescue stories resembling almost every action movie made from 1984 to 1992.
Streets Of Rage gave us kidnapped children and no convoluted romance.

Rival Turf gave us a burly man in a pink genie outfit hanging out on the street in broad daylight.
Streets Of Rage gave us a muscle-bound wrestler in an underground fight club.

Unlike its predecessors, Streets Of Rage 4 had almost no reference to the weird cyborg storyline that ruined Streets Of Rage 3. No enemies came at you with jet packs or giant clawed gloves. The game had one giant robot that didn't show up until the end, but it was controlled by a person instead of being autonomous.
Some blatantly ridiculous things in the series were removed, and the development team has been good about that since the stupid clown juggling axes in the first game.

Thank you, Sega. I waited over 20 years for a sequel and I couldn't be happier.