Its very hard to tell the difference between those two at 100% zoomed out, I could only notice at about 300% and I had to zoom in 800% to illustrate it here.
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This is compression artifacts so your output settings are set to compress the video, too much if you think it really looks that bad, however, my suggestion might be to ask yourself if it is really that bad and if anyone but you will notice.
Let's assume it really is that bad and you really
do need a fix. The old school would tell you to “let renderers render and compressors compress” meaning output from PP using an intermediate codec like ProRes on Mac or DNxHD/DNxHR on Windows. Then you can play the output video without compression to make sure it looks like how you want it to look. (Note there’s still compression it’s just far less than with a delivery codec like H.264.) This is a process called mastering.
Then once you are satisfied that the video has rendered out how you want it to look THEN you use Media Encoder to encode it to H.264 and compress it how you need it.
The new school would tell you all that isn't necessary and you are trying to make this harder than it is. They'd tell you to just use "H.264 best quality" as it is the golden standard of H.264 delivery output. For 4k that is 1 pass, variable bit rate, 80 Mbps target. This should be more than is really necessary. Sometimes it is best to just keep it simple ya know?
I think Media Encoder honestly gives most people way too many options. It doesn't need to be all that difficult to choose a codec. The only reason you would use all those other settings is if you have a really specific codec you have to use per a client specification or for broadcast or something.