There's no voltage difference across R1, so there's no current flowing through it, hence the ladder is broken somewhere.
Since you measure a voltage on both ends of R1 its connections are fine.
You can apply exactly the same reasoning for R2, so that's fine as well.
Since both ends of R4 are also at the same voltage (0V) its connections are fine too. (see note below)
Only R3 remaining, so the open circuit has to be there. Either R3 is broken (open-circuit) or it isn't soldered well.
the reading at the R3/R4 node is 0V, which could mean one of two things:
- you're actually measuring the ground voltage. In that case R4 is fine, as mentioned above
- but there's also the possibility that you're not measuring anything, because R4 is broken as well. In that case the node R3/R4 is floating and will read )v too.