The first part of the 4th song is now finished. After the last 3 songs were particularly slow (song 3 was probably the slowest song in a long time) I decided to make a faster song. Since I usually prefer my slow songs, it was quite painful at first to work with a fast tempo, but I didn't want to give up.
So when I reduced the tempo of the song slightly, I started to get used to the song - even if it still remained much faster. After initial experimentation with alternative bass lines (which only frustrated me) I added a fast bass line and extended this with a slow melody at the top.
It's not uncommon for me to work on a song and keep thinking to myself that it's okay, but not great for my taste, and I have doubts about whether it's worth releasing. Often it's just one note that I change, which suddenly changes the feeling and I realize that this song is definitely going to be on the album.
Very often I reach this tipping point when I start thinking about a vocal line. If the vocal melody practically comes by itself and every note actually sounds good in some way, then this realization comes in just a few seconds. I had this moment just before.