You thought Gen 3 was the worst? At least it brought in the facade Slaking, the Dragon slayer Milotic, Metagross, and a couple other B-D Tier Defenders. Gen 4 is even worse.
Togekiss: May be the only worthwhile defender in the Gen, it walls Dragons and Machamp and stays longer than Gardevoir but that's about it. Granted, blocking those Pokemon is already half the meta. Unfortunately it's ruined by our new generalist/tank/GOAT Metagross, whom resists all of its moves. (It resists a majority of the Hidden Powers too)
Drifblim: When this.... blob is one of the best defenders, there's really something wrong. Nothing against it though, it walls Machamp, it can hit Dragons, and H/SB can take sizable chunks out of health bars. Unfortunately, with enough rapid tapping, Machamp still wins. But maybe it'll take another 30 seconds, tops?
Hippowodon: It has okay defensive coverage, and well rounded stats. It's the equivalent of Milotic (At least defender wise) being a different typing, blocking Rampardos and Raikou with ease. Stone Edge can hit like a truck and Body Slam fires fast and quickly as well. Perfect to be put after a Gyarados! Check my other post about all that. (Stone Edge hits Moltres/The Big 3 Dragons hard) Unfortunately for it, it's fast moves suck.
And that's all for "viable." Here's the other Pokemon that are F-Z tier.
Gallade: Another Gardevoir esque wall except it doesn't double resist Machamp. It's okay, but it may not even get a Psychic out before Machamp can defeat him. At least he's not weak to steel.
Bronzong: A discount Metagross, it resists many types and has "okay" bulk.
Lickyilicky: Why would you use him if you already have BlissLax and ChansKing in your gym?
Bastiodon: Oh look, another Aggron! Machamp, I choose you!
Rhyperior: Just another speed bump for Machamp. It does well against a lot of other opponents though, but it doesn't gel well with the normal type tanks. (And remember it's also doubly weak to Water AND Grass)
Yanmega: Might annoy Machamp slightly with it's good attack stat and double resistance, but it doesn't do much else.
Mismagius: It doubly resists Machamp, and has above average bulk, but it can't hit it super effectively and it only takes maybe 5 seconds longer to beat?