I thought the same, but tried it for a month and it didn't work for me. Just as you get two Fridays a week (Tuesday and Friday) it also felt like I had two Mondays (Monday and Thursday). I just ended up hating Thursdays as much or more than Mondays, maybe because I was the only one doing it.

Also, sometimes I need a few days momentum to get something over the line and two days is often not enough. Anyway, just my 2p, more to the point I would totally jump at the chance to have a 4 day week permanently.