Bears repeting: IE8 is at < 8% global share, < 5% in many markets, and effectively 0% for any tech-related business. Google has dropped support almost a year ago. Mobile and Safari are way bigger concerns.
One site I run is a local news site (East Coast, USA) that does a fair amount of traffic (~10M pages, ~500k visitors monthly), and IE8 is still hovering around 13% for us, with IE7 at just over 2%. In fact, IE8 > IE9, by a fair margin. The good news is that IE share overall is down substantially.
Jul 2010: 74% Jul 2011: 65% Jul 2012: 58% Jul 2013: 34%
Foundation 4 has some caveats when it's being used with IE8 (http://foundation.zurb.com/docs/support.html).
Personally: I'm quite unfamiliar with Foundation so I'd go with Bootstrap 2 or 3. If I had to do it for work I'd go with BS2 because it's stable. Otherwise BS3.
note: IE8 is only beating IE9 right now because of automatic updates moving people to IE10 (finally!).
our lesson learned: it's not the quantity that is important here, but rather who is still (stuck) using older browsers - "don't bite the hand that feeds you".