Someone actually went back and rescored Sonic 4 to sound like it belongs on a Genesis and it works so well. This fan arrangement strips out the original's much-debated synths and rebuilds everything with YM2612-style FM tones.
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Splash Hill Zone suddenly feels like a proper successor to Green Hill when those warm FM bass kicks hit. Casino Street Zone gets this crunchy, bouncy treatment that channels Sonic 2's Casino Night perfectly. Even the Boss Theme Phase 2 has real weight to it now.
Mad Gear Zone Act 1 might be the standout, all gritty industrial FM. The whole thing is basically the version of Sonic 4 people wanted in 2010. Worth it just to hear what could have been.