The reason they use the term old version, is that they came out with the updated version of the 818 in 2013; the 828. The 828 has a few new features that the 818 doesn't have, but they also cut a few of the features that the 818 has that they didn't include with the 828, Go to the onkyo website to compare the specs, but for me I couldn't justify the price disparity when it came between both units, so I went with the 818, and I haven't been dissapointed! For $699 I don't think I could find a better receiver. I looked at Marantz, Denon, and Harman Kardon to compare, and for the price I thought the Onkyo was a better constructed, sturdier product at a lesser price.
No. There are two HDMI outputs, Out Main and Out Sub, but they are monitors (duplicates), not separate channels. The sub channel won't display onscreen menus. The intent is for systems that might have a TV and a projector in the same room, where you might want to use one or the other depending on source content (TV versus movie watching).
It will depend on the ohm rating of the speaker. The onkyo is rated 135 watts for all channels at 8 ohm, 180 watts for 6 ohms (IEC), and 230 watts per channel at 6 ohms (JEITA). I am not completely sure as to what the wattage rating is for a 4 ohm speaker arrangement. Hope this will help you with your decision, I will let you know with the more familiarity I gain with this receiver, I become exponentially more impressed with the sound clarity I get with this receiver, and it has plenty of clean power, I love to hear all genres of music at a higher volume level than most. I will tell you that I only run monster cable for my components, with banana plug connectors for all speakers, running a minumum. Of 16 guage for speaker wiring, and even running some 12 guage, I know that this cabeling definitely increases my signal clarity. I recommend putting the investment in the hook ups, if you are planning to have a better receiver; it will enhance your listening enjoyment. To test your legit sound quality, I recommend you test the sound with a CD that has good sound production, like the new Dream Theater CD in 5.1 surround, or something as dynamic as a Transatlantic CD, or for just something that is just in your face powerful as Metallica's- Death Magnetic, that will show you that you got your money's worth from this receiver.
No, it doesn't pass through 4k sources.
Note: 4K video playback requires a 4K-compatible TV. HDMI pass-through of 4K content from source devices is not supported. 4K upscaling of analog video is only available at 480i.