Is there any way of figuring out when a photo on my iPhone4 has been taken, without first importing the photo to a Mac or PC?
Since the available photo options within iOS are limited, I don't think it's possible without any 3th party software.
King Camera is such an app. It shows some meta data about every photo you took with the app.
If you are looking for a free option, have a look at EXIF Wizard. It displays all EXIF attributes (including date/time) and also can use the GPS coordinates in a picture to show the location on a map.