I should say fourth day rather than fourth night, as the process makes everything a big, large day.
Third day has been the toughest, with a few moments of acute tiredness, with more important deviations from the sleeping schedule. I realized that micro-naps of 5 or 10 minutes also help to clear a feeling of extreme sleepiness. Still, most of the time awake has been spent completely normally. As I also work most of the time (on some task or some other of various difficulties, including writing this blog on the easy end and doing research on the difficult one), it is also conceivable that tiredness becomes occasionally more acute. This was also the case in monophasic phase.
Today is smooth and easy again. The fourth night was also the occasion to see how this would affect Elena, who was back from abroad. She had only mild opposition from me doing this. Actually she got interested enough to wonder if she should try. I could get up quietly from my core nap, between 01:19 and 06:09, without waking her up. It was a bit later because we had watched one episode of Attenborough's Life series. I didn't fall asleep which is often the case when watching something in bed. Either the scheme is indeed working or the show was captivating enough to keep me awake.
So far my intention is to carry it on further as much as possible. I would also like to compile more systematic information from the literature. So far I only got empirical evidence from eager and curious individuals but who seemed to have no academic credentials related to the topic. I found only one negative assessment denouncing the technique. I will need to learn about such points as sustainability, how to stop, coexistence of various schemes, possibility to alternate monophasic/polyphasic phases, etc., or I will have to discover all this by myself. Possibly the painful way.