I think I have a possible explanation for why the "4" in "4o" is sometimes written with a straight line and sometimes with a softer edge (like a "c" shape).
In the late 1300s and early 1400s, the numbers styles were changing.
In the 1300s, they wrote the 7 like a mountain and the 4 like EVA-l. It was rounded. Gradually, during the 15th and early 16th centuries, the 4 evolved into the 4 we know today, with straight angles but I have noticed, in a number of manuscripts created during the transition period, that some of the scribes who were writing the new-style 4 (the straight one), would occasionally revert to the old one. Sometimes they crossed it out and rewrote it, sometimes they straightened it, and sometimes they didn't bother and wrote both styles interchangeably.
So, if it's someone who learned old style (rounded), but adapted to the changing new style (straight), it's quite possible that they would occasionally slip and write it the way they learned it when they were younger.
I'm not saying the "4" is actually a 4, but since the VMS glyph is almost the same shape and written much the same way, the same habits of the hand might apply.
In the late 1300s and early 1400s, the numbers styles were changing.
In the 1300s, they wrote the 7 like a mountain and the 4 like EVA-l. It was rounded. Gradually, during the 15th and early 16th centuries, the 4 evolved into the 4 we know today, with straight angles but I have noticed, in a number of manuscripts created during the transition period, that some of the scribes who were writing the new-style 4 (the straight one), would occasionally revert to the old one. Sometimes they crossed it out and rewrote it, sometimes they straightened it, and sometimes they didn't bother and wrote both styles interchangeably.
So, if it's someone who learned old style (rounded), but adapted to the changing new style (straight), it's quite possible that they would occasionally slip and write it the way they learned it when they were younger.
I'm not saying the "4" is actually a 4, but since the VMS glyph is almost the same shape and written much the same way, the same habits of the hand might apply.