I'd say no.. go fill the other days in the week with more clients. Keep the flexibility to say no. My 2 cents. 20+ years of consulting experience working mostly with single clients or small business.
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.
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.