From my perspective as what you might call a super-senior, I find this process itself a giant red flag. It is aimed towards recruiting juniors.
I have done enough contracting that before you have finished the sentence, I have already worked out in my head the opportunity cost of 4 non-billable hours. And in return I get.... nothing. Now while it is true that 4 hours of on-site talking to people is still 4 hours out of my life, but at least then I get to meet the people I would be working with and have an opportunity to ask about the company. And you are showing respect for my time by spending your time getting to know me.
And what do you learn from a 4 hour homework assignment that any undergrad should be able to complete, when you can see from my resume multiple L5/L6 positions at different companies? Nothing at all about what you want a senior to actually do. The senior you need is the person who can mentor the juniors through a 4-month long (not 4 hour long) project. Review their architecture, guide it in wise directions. Review their code. Raise the bar on unit testing. Teach the entire organization about better validation methodologies. Create realistic schedules and rational work-streams. Evaluate new technologies.
As a candidate, my interpretation of your process is that I would be working for management that can't think about problems from first principals. You can't hire a senior because you aren't looking for a senior because, as evidenced by your process, you don't know what a senior does.
The first thing you need to do is articulate to yourself what you want a senior to bring to your organization. Structure your interview process to try to uncover those skills and behaviors in a candidate.