Indeed, the v4b download appears to contain only some xml error message response - it is not a zip file, it just has that extension. Apologies I didn’t notice that earlier.
Edit: looks like this is not the only dead link on Sony’s website:
Also seems unavailable from Wayback.
Looks like you’re SOL, must be some issue on Sony’s server. At any rate, you can still use the v3a files and simply flash v4b over the top later if you manage to get ahold of it, it shouldn’t affect your existing installation. Perhaps some other users still have a copy of v4b on their hard drives they could share with you.