No one actually knows how to lculate what speaker cable they need

b4icu,
we are thinking of changing from 4 AWG to 2 AWG to squeeze a little bit more from the amps. What is your opinion?
We are looking for better connectors from Furutech. Most connectors max wire 6 mm only 😰

" we are thinking of changing from 4 AWG to 2 AWG to squeeze a little bit more from the amps."

- But wait, isn’t the "correct" cable size based in the magic formula that includes the speaker specs. & the amps DF? If you go to a different size, would that not violate the prime directive?
- Who is the we? Do they have the same amp & speakers that you have?

"What is your opinion?"

- How can an opinion matter? Remember the formula tells all ...

Mr. khiak
For your equipment, going from #4 to #2 won't do much of improvment.
As per the AWG table: https://www.powerstream.com/Wire_Size.htm
The chnge in resistance is not that much from  #4 to #2.
For the banana plugs, I recommend on this:
https://www.ebay.com/itm/8x-Nakamichi-24k-Gold-Plated-Audio-BFA-Banana-Speaker-Plug-Screw-Cable-Wire...
and a short #8 to connect between the #4 and the banana plug.

Please be kindly reminded for my requests. Thanks. 

Do NOT use Nakamichi bananas. The advertising on these are false, they are not gold plated copper, they are brass, no copper to be seen. See link below, I cut one open. Use the higher end Furutech or Cardas bananas. Also, it would not surprise me the 4ga. or 0ga. wire you are using are not pure copper, at low prices battery or welding cables might be an alloy.
https://photos.app.goo.gl/yHvsgifGyJVwy5Nh9