A four-day-a-week worker here.

I don't know what you exactly mean, but my personal experience is exactly the opposite. I worked for a startup as a founding engineer, just 4d/w (the CTO was crazily open-minded), and I was never so productive. Doesn't matter that the others were working 5 days, pushing more; it was my responsibility to keep up, and it worked pretty well.

Same now, working for a company with the same arrangement.

And no one is or was "losing."