An SBR of 4 would imply an extremely low contrast scene. The scene I have referred to above was definitely contrasty, much more than normal. Even without using a meter I can observe (and remember, as I shot this scene yesterday) that there was a significant difference between high and low values.
As regards my technique, I have been using this metering method since taking a BTZS workshop back in 1996. It has not let me down yet. Recently I have decided to switch to Pyrocat HD and Efke and Classic. But rather than play with a densitometer I thought it would be more fun to take actual pictures (2 to 4 shots same scene, same exposure) and experiment with dilutions and times. This is why I have been interested in specific instances of high SBRs (plus the fact that Stockholm archipelago is quite contrasty during clear, sunny days). I initially started with extensive BTZS tests for TMX and HP5. Using this metering method together with the data and charts for development times and SBRs matches well the rule of thumb ("...plus 5") that you mentioned.
I just do not see how what I saw yesterday translates into an SBR of 4. Not with that kind of sunlight.