Here's an easy way to break zoom in and out capability in IE7+ in your ExtJS4 application:
There's a <meta> tag (the ones that go into <head>) that was recommended to me for the sake of reducing IE testing efforts:
<meta http-equiv="X-UA-Compatible" content="IE=8"/>
The idea here is it forces many IEs to act all in IE8 mode, so you can basically do quick tests in just IE8 and still get a good test coverage. However, it took me some effort to figure out that this breaks re-sizing capability of a Viewport on zoom in IE.