4o

[-JKP-]

Don't bother: I've found it now.  It's actually an common abbrev.  Thanks for the help.

I'm about 200 posts behind so I didn't see this request until after you had found it for yourself (yes, it's often used to indicate quarto in Latin texts, even before the pointy-4 came into regular use).

But I have another example, something that's not uncommon in 14th- and 15th-century texts. In this example...

                  

it's easy to read it as a four, especially if it is followed by a letter that attaches to the top of the right-hand stroke (and makes it look more like an "o") but it's the letter "p". Sometimes the loop on the p is rounder and almost completely detached from the descender and looks very much like the VMS 4o.

I sometimes wonder if this shape served as inspiration for the French code and perhaps also for the VMS glyph-shape 4o. Note that in the French code, the 4o shape stands for "p".