I wouldn't exactly call Sonic & Knuckles "Sonic 4" per say but a very solid addition to the series. It has an interesting plot, great cutscenes, superb graphics and amazing sound! Released in late 1994 for the SEGA Mega Drive, Sonic & Knuckles is basically Sonic 3: Episode II. It continues after Sonic 3's ending which asked us many questions like, why is Floating Island still on the ground and why are there some areas of the levels that Sonic or Tails can't get into? Well, Sonic and Knuckles answers those questions (Note, this is a review of SONIC & KNUCKLES, not SONIC 3 & KNUCKLES or KNUCKLES IN SONIC 2, this is a review of the stand alone game with the funky cartridge with "New Lock-On technology!!".)
The control for S&K Is typical Sonic really. Run is D-Pad, Jump is A, Spin Dashing is A+Down but the control is slightly altered when you play as Knuckles the Echidna. Knux (as I Shall call him) can glide by pressing jump in mid-air and can also climb on walls which answers the question why Sonic or Tails couldn't get to some of the areas in the levels.
S&K has the exact same graphics as in Sonic 3, so all there is really is new levels, which look really good and two of these levels are re-used in the Anniversary title, Sonic Generations for PS3/360/PC/3DS. I really like the detail in some of the levels, which is why I think S&K is the best looking old-school Sonic game by far, even better than Knuckles Chaotix on the 32X.
The sound is top-notch. Howard Drossin and Jun Senoue have composed what is probably the best soundtrack on the SEGA Mega Drive. Sky Sanctuary Zone, Mushroom Hill Zone and OF COURSE the Doomsday Zone are all the real standouts here, but it's an overall great soundtrack.
Story-time guys, so tuck in. let's start with Sonic's story, which begins immediately after Sonic 3.
The Death Egg has crashed but Floating Island is still grounded, landing in a volcanic crater. Sonic and Tails rush their way out of the Launch Base and head towards Mushroom Hill to see if Robotnik survived the crash. Sonic whilst seeking Robotnik is looking for the Chaos Emeralds (or Super Emeralds if you already got all 7 Chaos Emeralds) and once again begins conflicting with Knux who believes he is trying to steal them...again. Robotnil amazingly survived the Death Egg's fall, and reveals his true plan: Steal the Master Emerald, the key to Floating Islands.....floatiness. Realizing he was tricked, Knux decides to help Sonic and Tails. After getting through and Destroying the Death Egg, Sonic has one more thing left to do; finish Robotnik off and take back the Master Emerald. After becoming Super, Sonic has a one on one battle in the Doomsday Zone with his nemesis, which he defeats and Floating Island floats again thanks to the power of the Master Emerald.
Now Knuckles story.
Our friend Kucx was sleeping one day when all of a sudden his peaceful life is disrupted by one of Robotnik's last robots (which means this story must take place after Sonic's) called Egg-Robo. After chasing the Egg-Robo, Knux encounters Mecha-Sonic who appears out of nowhere and destroys the Egg-Robo. Mecha-Sonic steals the Master Emerald but before he can leave Floating Island, Knux shows him whos boss and returns the Emerald back to it's pedestal.
Really addicting I must say, can play it over and over again, and with Lock-On, it just makes it even more enjoyable.
Not too hard, but quite challenging near the end.