they all will work it's just how fast, the 1.1/2.1/3.1 are almost identical appart from cpu speed.
the 2.1 8c 3ghz is slightly faster than the 3.1 2.8ghz 8c just because it has the extra hrz
This thread is littered with misinformation. Please do not pay any attention to what ^ said.
First off, the 1,1/2,1/3,1 are not 'almost identical'. Whatsoever. The 1,1 and 2,1 are close and considered the fairly obsolete range; but the 3,1 is a significant upgrade. If you have the option, never go lower than a 3,1. There is upgraded architecture, faster bus, faster memory, significant pci upgrades, 64 bit kernel support, etc. And the 2.8ghz 3,1 IS faster than the 2,1 3ghz. The extra ghz do NOT make the 2,1 faster. The Harpertown vs Clovertown processors are a no brainer.
And for editing 4k video - it HIGHLY depends on the video you're going to be editing. Is it gopro 4k footage? Cell phone 4k? Or is it 4:4:4 4k shot on a red, or blackmagic, or something like the a7s with a shogun? You can edit gopro 4k on a somewhat basic 3,1 setup easily, but if you're editing pro-sumer to pro-level 4k footage, it will take some upgrades on any Mac Pro. You will need quick drives to access the footage; if the bitrate is significant, you'll probably want an ssd raid setup. That alone will make the 3,1 handle it fairly okay. A decent GPU helps as well. From your posts, it seems like you mainly just want to be able to cut it without huge hickups; as render time doesn't matter to you. Your best/cheapest solution might be a 3,1 with a 240gb SSD in the PCI slot, and an additional 7200rpm drive. Assuming you do mostly short projects; the 240gb should be just enough; and when you're done, dump it all onto the 7200rpm drive.
My personal recommendation would be to go with a 4,1 or 5,1 though; if that's an option.