I refuse to spend the time and bandwidth to upload 4K at those prices and commissions.
But, that said, I still can get about $1K - $2K per month with my existing portfolio, so...
I decided last year to submit some new material to iStock, but I now downsize all my 4K to H.264 HD files specifically for iStock. The files are more compressed than I care for, but they pass the iStock reviewers. And the buyers on iStock who are used to paying little for their stock will only get my highly-compressed H.264 HD files.
That's my justification.