For OU, Fire/Ice/Grass/Dark misses 10 Pokemon: Blissey, Chansey, Conkeldurr, Gyarados, Heatran, Infernape, Jolteon, Ninetales, Tentacruel, and Volcarona. But I think substituting Ground over Dark hits more useful Pokemon; it misses a bunch of Psychics (Alakazam, Deoxys-D, Espeon, Reuniclus, Meloetta, Mew) as well as Gengar, but it hits key Pokemon like Ninetales, Heatran, and Tentacruel. Overall, it misses 12 Pokemon to Fire/Ice/Grass/Dark's 10.
Taking Fighting over Ground gets Blissey and Chansey, but loses Jolteon, Infernape, and Ninetales (and Toxicroak, if you're not counting it as weak to Fire). You can swap Grass for Electric; you'll hit Gyarados but will miss Gastrodon, Rotom-W, and Mamoswine. Grass/Ice/Fighting/Ground misses 14: all of the ones listed (minus Blissey and Chansey), along with Scizor, Genesect, Skarmory, and Forretress.
It's interesting how types cluster in OU; Ice/Fire actually has better coverage in OU than Electric/Ice, despite being resisted by the entire Fire and Water types; it hits 27 Pokemon super-effectively. But I think the two-type coverage award goes to Ice/Ground, which hits 28 Pokemon super-effectively (with no overlap). Grass has more utility than Electric, because despite being heavily resisted, it hits the abundant Water- and Ground-types, and there are more of them that are weak to Grass than Electric. Of the four types listed, here's how many Pokemon each type hits that aren't hit by any of the others:
Fire: 7
Grass: 9
Ice: 9
Ground: 5
Dark gets a lower number of Pokemon missed, because it hits 7 exclusively compared to Ground's 5.
