Do you really wanna know

Rubbish! The 3400G has HDMI 2.0a and Display port 1.4, so 4K support is native... The question is however, what do you want to run on it? Will it play Crysis in 4K? Probably yes! And CS GO so long as you don't mind 'lower settings' and 60Hz or lower!
 
But overall, it depends on the game/title...
 
Ignore any replies that speak in absolutes!

Am really curious about the question...
 
" I want a cpu that works best at lower resolution gaming" - But you have an i7 6800K? That's already way way beyond lower resolution gaming.
 
A 3950X is a top 'o the range home/semi-pro video encoding CPU, so you're way way above your desired spec.
 
There is a lot of pants and nonsense talked about the 3950x. That CPU is many things, but what it most definitely isn't, is a low res gaming CPU. Ignore anyone that say differently, and just watch the reviews of the 3900X...
 
There are far too many 'gamers' that just don't understand what CPUs do, or what they are designed for. To meet your needs as you describe, a 3600 would be more than adequate, as would your existing set-up!
 
Maybe if you say whats wrong with your existing set-up, it would be easier to decide, but for low res gaming and light encoding, you're already way way over spec as I said...

This is not true. At 1080, the 3700X is close to the i9-9900K in some titles, even marginally better in some. But in most the i9-9900K is more than marginally better. In fact so is the i7-9700K.
 
What is true is that the 3700X is about US$30 less expensive than the i7-9700K. If one puts it in a B450 motherboard it becomes a good choice for a gaming system.
 

Am really curious about the question...
 
" I want a cpu that works best at lower resolution gaming" - But you have an i7 6800K? That's already way way beyond lower resolution gaming.
 
A 3950X is a top 'o the range home/semi-pro video encoding CPU, so you're way way above your desired spec.
 
There is a lot of pants and nonsense talked about the 3950x. That CPU is many things, but what it most definitely isn't, is a low res gaming CPU. Ignore anyone that say differently, and just watch the reviews of the 3900X...
 
There are far too many 'gamers' that just don't understand what CPUs do, or what they are designed for. To meet your needs as you describe, a 3600 would be more than adequate, as would your existing set-up!
 
Maybe if you say whats wrong with your existing set-up, it would be easier to decide, but for low res gaming and light encoding, you're already way way over spec as I said...

Pull the graphics card and see what the on board ports do without a dGPU.

I'd suggest not caring about 1080p60 first (as well as about crazy bitrates and presets). Watch EposVox on youtube, there are a lot of pieces of advice on streaming and such.
The best way to stream right now is either with Ryzen or new NVENC on Turing cards (GTX 1660/1660 Ti, RTX 2060 and better).

Before you get any more daft advice, please watch this, before proceeding:
 
Steve from Gamers Nexus:
 
 

For budget Im a huge fan of the Cougar MX330-X variant.  I was so interested in it (to compare to my CoolerMaster HAF XM - which stands for HIGH AIR FLOW) I bought one and swapped my RR1 rig into it and my temps are what I would expect.  Entire front mesh, side panel intake, top and rear exhaust with a basement for $42 shipped?  Its a great deal imho

Before you get any more daft advice, please watch this, before proceeding:
 
Steve from Gamers Nexus:
 
 

Rubbish! The 3400G has HDMI 2.0a and Display port 1.4, so 4K support is native... The question is however, what do you want to run on it? Will it play Crysis in 4K? Probably yes! And CS GO so long as you don't mind 'lower settings' and 60Hz or lower!
 
But overall, it depends on the game/title...
 
Ignore any replies that speak in absolutes!

Have you considered X370 boards? They have way way better VRMs' than the boards you suggest, and are now much cheaper than they were...
 
You might be better served by X370 than B450?