Hi Carol, sounds like a very interesting book you've got there. However, the blade(s) used by the Ripper had to have been at least six inches in length, so the 4 in. blade you described wouldn't have cut it (pun not intended).
What immediately struck me about a folding knife of this description is that it functions as a knife with two different blade lengths. Unfolded, it has a 7½-inch blade. Folded, it has a 4-inch blade exposed.
Assuming that JtR killed Tabram, the shorter portion of the blade might do very well for the injuries found on her. This would also fit with the assumption that the killer attacked Tabram on impulse, unpremeditated, simply pulling the knife out of its sheath and stabbing frenziedly with the short piece of exposed blade. For subsequent, planned killings he could have unfolded such a knife to inflict the injuries found later in his career.
The knife you describe, Carol, sounds like a reasonable implement for JtR to use. I like the fact that it has "a best solid leather sheath" which could, no doubt, be hung from a belt and therefore hidden by the jacket that virtually all men, regardless of economic circumstance, wore in the LVP.