Trailer looks ok. Models are somewhere between the newer Street Fighter games and KOF games. More important is the animation. It looks good here but the cut for trailer camera angles obscure any clear gameplay so it's hard to tell how good it actually moves.
How hard is to take the best of Super Double Dragon, Double Dragon Advance and Double Dragon arcade with the production values of Streets of Rage 4?
Quite difficult. That would require them to combine and rethink the mechanics of all 3 games into 1 that stays true to all 3 and rework with the enemy placements and level designs that will suit the new mechanics.
But the hardest part would be animating the characters. Not sure if you know but quality 2D animation is one of the most time consuming things you can do and there are no shortcuts (cutout/flash/puppet rig animation is obvious and not comparable at all). The animation on SR4 is high res 2D hand drawn cleaned up full animation with excellent posing, anatomy, proportion and design, and they animate with weight and impact. Animators capable of drawing stuff like this is extremely rare, rarer still they work in video games. Unlike 3D where all you have to do is model a character once, rig it up and move it, in 2D every frame is hand drawn, so every instance of movement per character is an entirely new model from scratch. Unlike pixel art there is no grid to guide the line drawings and yet they have to be precisely in the right place from drawing to drawing, and there are so many lines given the fidelity. It's something of a dying skill outside of Japan.