What's the character between 3 and 4 in the table at the end of the article ? Does anyone know ?

It's 4. The columns are the digits 1, 2, 3, 4, 5, 6, 7, 8, 9, 0 And the rows are the character used for that digit at that point in time.

At some point later 5 mutated from looking like 4 to as it is now.

Also worth comparing with the numerals used today in Arabic:

http://userpages.umbc.edu/~samir1/620_Project_2/L3T1_1.htm

It surprised me to see the digit forms in the table at the bottom in use so late, since roman and italic types from the late 1400s look so familiar, but then i realized that the various samples I've seen likely had no digits at all. The earliest I can find quickly is from 1501†. Anyone know more? It seems plausible to me that these early roman & italic types established a standard by the newly wide dissemination of these forms, as they did for certain punctuation.‡

† http://en.wikipedia.org/wiki/File:Livro-Aldus.jpg

‡ http://en.wikipedia.org/wiki/Semicolon