The fourth console generation started off with the release of NEC's PC Engine, or the TurboGrafx16 if you're American, which used an advanced graphics chip capable of producing 16-bit graphics. For comparison, the NES could only produce 8-bit graphics. The console sold well in Japan, but in North America, it was always one step behind and it didn't even get released in Europe.
Sega transformed their arcade system board into its next console, which was released as the Sega Mega Drive in Japan (1988) and in Europe (1990) and as the Sega Genesis in North America (1989). Sega boasted its console's graphical superiority over the NES and used it to create incredible ports of popular arcade games. Sega even called out Nintendo's inferior hardware and used it to boost its own sales with its effective "Genesis does what Nintendon't" marketing campaign, allowing Sega to gain a strong foothold in the console industry.
Nintendo responded to Sega's "trash-talk" with the release of their newest system, the Super Famicom. It was released in Japan (1990) and in Europe (1992) as the Super Famicom, but it's name was changed to the Super NES or SNES in North American (1991). The SNES boasted its superior graphics and flexible sound chip over the Genesis, but the Genesis had a faster CPU and sound chip. Sega knew that it need a mascot that could compete against Nintendo's Mario, so they released Sonic the Hedgehog in 1990, which SPIKED Genesis sales. Nintendo dominated the industry in Japan, but Sega won Europe and the two continued to battle it out in North America, with Nintendo ultimately claiming victory, but only by a small margin.
In 1991, SNK repackaged the NeoGeo MVS, an arcade system, as the Neo Geo AES home console. The system had graphical superiority over other fourth generation consoles, but the console and its games were far too expensive, which caused its limited success. NEC released many different Add-ons to their TurboGrafx16, including the TurboGrafx-CD and the TurboDuo. Sega introduced Add-ons to their own console, with the Sega CD and 32-bit drive. SNK decided to try their hand at CD technology with the release of the Neo Geo CD.
Nintendo released the Game Boy in 1989, which went on to dominate and solidify the handheld console market. Atari entered their Atari Lynx into the sprouting handheld market in 1989. It featured colored graphics, a backlight, and superior link-cable multiplier. Sadly, its sales were lackluster, due to its extremely poor battery life and its small library of games. Sega decided to jump into the fray with their Sega Game Gear in 1990; however it suffered from the same problems as the Atari Lynx, but it did have a much larger library. NEC released the TurboExpress in 1990, which was just a handheld version of their TurboGrafx16, but it sold poorly.