That's not easy to answer. It depends on the frequency a user needs to work with QV.
A session CAL is quite expensive and only pays off, if the license is released by the users quite frequently (i.e. if the license it's locked by e.g. same 4 users all day, it's probably cheaper to buy named user CALs or even document CALs, if the user only requires to see 1 document).
I suggest that collect some statistics / assumptions on the document usage across your users, then meet with your QV sales rep or QV partner to discuss possible license scenarios.