[hackerfromthefu]

This is 100% the correct answer.

Their offer is BAD for you:

One single client puts you in a situation of risk when they pivot or get acquired or otherwise cut your work.

4 days a week makes it harder to have time for another client.

Less money for more work, ludicrous

So instead

Go and find another client, or better yet two new ones and when you have multiple clients prioritize the work for the best paying clients while others can wait till you have gaps.