full frame or keep T4i
I think you are a good candidate for the 6D. Like the poster above, I'd wait 6-8 months for the price to drop $200-300. You'll have to sell the 18-135 as it is EF-S. The 24-105L is a nice replacement, but not quite as long obviously. It is faster though.
There is a sale on the 50mm f/1.4 right now. That pairs great with the Rebels. Going for $299 at B&H.
Use this link if you buy since it helps support CanonRumors, which is a great resource.
You are better off buying quality lenses. You can always upgrade bodies later. The t4i is a great camera and has better AF than the 5D2 since the outer points are cross type.
The original 5D is an amazing camera and will allow you to stand closer to your subjects with your 85mm and get the same framing. This will cause the background to even further fall away.
The colors/skin tones of the original 5D are unmatched to this day for a DSLR. It won't have any better ISO performance than your t4i, but it doesn't have banding either.
If you buy it, I would keep your t4i since it will be more versatile and will be better for sports. The 5Dc doesn't have live view or a movie mode.