^ Having been on the receiving end of Hallolet's Carnifexen, I'd strongly suggest pinning a Knight with a screen of Gaunts, and then charging in with the Fexes. If he's shooting your Fexes, you'll be getting cover saves from the Gaunts in front. Assuming you're using the guy that gives everyone Shrouded, then your Fexes have an MEQ cover save on the way in.
That's annoying. A lot of annoying. Keep a couple of the Shroud-Boosters spread apart, to ensure they don't both get snagged, and you're laughing. If you can hop along in ruins, you've got a TEQ cover save to keep him alive, and if a Knight is using a pair of BC shots to try and get one of them, that's eating 25% of his firepower. You only need a couple of turns to close the gap. 3 Tops.
To specifically tailor, a few small Gaunt Squads [faster is better, Termagaunts, I think?] that can run up, get stomped, but hold a Knight in place to allow you to charge. I mean, a hypothetical 6 Lascannon HoW hits should net a trio of HP... and that's before the punches start flying. You're apt to get hit a couple of times at MEQ Initiative, so you'll likely loose a Fex, but the two others should finish the job.
Anything with Armourbane Claws will go a long way.
Knights aren't an army like any other. They're a game changer and "standard" tactics don't work. A pair of Flyrants can't be touched by Knights, so just circle them all game long. I can't remember a Tyrant's BS, but I think it's better than a Marines?
If so, you're going to get 10 hits on average, and most of the time you'll get 2 glances out of that. A pair working together should average 3-4 HP per turn, dropping a bit for the Shield. The two Flyrants should take out a Knight every couple of turns, most of the time. They should normally get 2 of the Knights by themselves over the course of a game, assuming you can keep attacking from different arcs.
I don't know if you'd use a pair of Flyrants in a typical 1500 point game, but 4 Knights isn't typical.