What she will gain mainly is a much faster device and a much better camera. From the 4 to the 4S to the 5 to the 5S, the phone basically doubled in speed every year.

Siri isn't supported by the iPhone 4, it was first supported by the iPhone 4S. Of course, given your ambivalence towards the device and her lack of technical clued-up-ness, she may well have a 4S already

Siri is pretty much the same on all devices that it is supported on.

If she does upgrade, bare in mind that the resale price on her aged phone may be higher than you'd expect. For example here in the UK a 16GB iPhone 4 gets you £90 at one of the "we'll buy your phone" sites.