For people who still can't use Optifine, even with the FML patch - it's probably caused by WorldCore, which is included by Biomes 'O Plenty. It's been documented here:
https://github.com/Glitchfiend/BiomesOPlenty/issues/135
And here:
http://www.reddit.com/r/feedthebeast/comments/1sgay9/horizon_pack_crashing_when_using_optifine_fix/
So go to MODPACKNAMEHERE\minecraft\mods\1.6.4 and delete the "WorldCore-universal-1.6.4-1.1.0.5.jar" file. Then start Minecraft. The file gets redownloaded during the launch process, so you're not breaking anything, but this causes enough of a delay for Optifine to initialize right.
Now someone figure out how to make Horizons work with Minecrift.