I was always under the impression that most people care much more about content than board software. Have you considered that maybe your board is simply offering similar content that can be found elsewhere and thus that's why you're losing traffic?
I mean, some of the biggest boards in the world continue to use ancient versions of phpBB and vB have don't have the slightest dip in traffic. So while it's possible vB4 may have somehow caused your site's traffic to go down, I doubt it's the main reason.
Whilst people do come for content, software can and DOES make a difference.
I suspect most big boards understand this, hence their sticking with what 'works' for them!!
When I converted from vB3 to IPB3 I had a lot of people walk away. I made mistakes, like completely changing the skin for example, and a number of those people will have been on the edge of leaving anyway (and the software change just pushed them over), but quite a lot of people didn't like the change and didn't persist with it long enough to get used to it.
I've no real way of getting those people back and just have to concentrate on making the best of what we've got and building the numbers back up naturally.
I'd also run the site from two domains (one was unrelated and people could use it to access the site from work because the cookies and paths in vB3 weren't domain specific) but because IPB doesn't support multiple domains a lot of postings were lost because people had to post outside of working hours (
who doesn't like to visit their favourite forums when they're on a break, eh?).
I also had a problem with people spreading their content over the forums, chat, blogs, gallery, etc. so the forums looked like ghost town's some days in comparison to our previous vB3 forums-only set up.
All of this despite careful planning, beta testing with large groups of regulars, and taking months and months to test and trial the new software.
I've learned from the experience, but the point in all of the above is that change - any kind of change - DOES have an impact on your site and membership.
I know vB4 is still a vB product, but it's a
different vB product.
One, it seems, that your users (
and possible Google) feel is different enough to re-appraise!!
Cheers,
Shaun