You might check this by using Browsersize at a Glance on an iPhone. This simple webpage will give you results like this:
|Safari on iPhone 4s, iOS 7.1.2|
|Chrome on iPhone 4s, iOS 8.3|
However now I wonder why Apple states that the iPhone 4s has more pixels than in the specs. Today I got this result and I'm puzzled.
|Safari on iPhone 4s, iOS 8.3|
Why do I now get the default width? The only thing I have done is updating iOS, so I wonder what I have done wrong that the default width is shown. Perhaps this is a bug.
I know this is a technical issue, so it is good to know that I have read Apple's Configuring the Viewport and I'm always using this code
<meta name="viewport" content="width=device-width" />
in the head section of the webpage.
By the way, I also wonder why there is a difference between Google's Chrome and Safari. I suppose that they are based on different versions of webkit.