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Here is the response given to me by Phil a couple of years back:
"You obviously can't measure a 4-stop SBR directly but it's certainly possible to assign a range of less than 5 to your subject if you want to treat it to much higher than normal development. The "5 stop minimum" is based on the incident meter dome's 18% transmittance which is equivalent to 2-1/2 stops less than 100%, and (when doubled), perhaps coincidentally, closely matches the typical reflectance range of flat copy, such as a magazine page with "black" ink on "white" paper. In other words, reflectance readings of those extremes of tone are usually in the vicinity of 1.5 which is equivalent to 5 stops.
It's reasonable, therefore, to assume that any 2-dimensional, uniformly illuminated subject — (and this is important) that you wish to reproduce with realistic contrast — can be considered to be a 5-subject, regardless of its apparent tonal range. If its a normal full-scale subject, it'll be reproduced that way in the print; if it's a low-contrast subject (gray on gray) that's how the print image will also look.
But that may not be how you'd like to interpret it. For example, you might want to photograph a petroglyph of brown pigment on gray rock of similar value. Using a 5-stop (or greater) range will produce a very low contrast image so you might arbitrarily treat it as a 4-stop subject which will greatly increase development time and therefore increase image contrast significantly.
The ExpoDev Palm program is set up to make this possible with the Incident metering method (not the ZS). You can take a single "low" reading to establish the exposure then arbitrarily enter a "high" reading that's as much as 2 stops less (corresponding to an SBR of 3 stops). This is very strong medicine and it's very likely that common materials won't be able to produce that degree of contrast, but if you have a film/dev combination that will permit it, you can enhance image contrast dramatically.
Incidentally, you can check your materials to see if they're capable of working in that contrast range by dividing your paper ES by the SBR. For example, if your ES is 1.0 and the SBR is 5 stops (1.5) the appropriate average gradient is 0.67, which most popular materials will handle easily. But if you decide on a 4-stop SBR the result (1/1.2=0.83) may not be within range, and a 3-stop SBR (1/0.9=1.1) is more than most common materials can handle, although there are a few exceptions. If you're using the ExpoDev program it'll warn you if your chosen input values exceed the materials' capabilities."
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