I just did these walks using his Lisbon guide/audioguide on the app the other day. It didn't take me four hours, probably more like 3.5 for the entire Baixa/Bairro Alto/Chiado walk. However, I didn't go in any of the suggested stops except for a church or two. I didn't ride the Elevador da Gloria. I did have to back up the recording and or backtrack in a few places where the directions weren't very clear, or I had passed where I was supposed to turn. I also stopped and sat down for a few minutes here and there, and at the funicular, we had to wait while the driver told off a bunch of inebriated women that wanted to ride. (They were all smoking or vaping, and one woman got on without paying, so he made her get off and shut the door in their faces.)
When I got to the end of the Baixa section I thought about calling it a day and resuming the next day, but I was staying in Alfama so I would have had to trek all the way back uphill. Also, looking at the map, I felt like I was more than halfway done. I just sat for a few minutes in the Sao Pedro de Alcantara park, then resumed. The Bairro Alto/Chiado part is mostly downhill and seemed shorter, even if it wasn't.
Unless you are staying right in that area, I'd recommend doing the whole thing in one go. There are plenty of places to stop and sit during the walk, and then carry on. I am in pretty decent shape and the thought of making my way back up to that spot to resume was a little daunting, so I powered through.
I did his Alfama walk the day I arrived, skipping going inside the castle and starting at stop 4, Castle Town. That took maybe an hour and a half; because that tour is not recorded I did more stopping off to the side to read. That tour starts at the top at the castle and is all downhill.