theodorn to see if we could plug her acoustic guitar (not electric) to my Focusrite Scarlett 2i4.
A lot of good input has already been given. Just to make sure, I'd like to add:
Because it's an acoustic guitar, it will probably have a piezo pickup. Piezo pickups have an utterly high impedance and produce an utterly tiny wee signal, which is why instrument level inputs often cannot use their signal. It's just way too weak.
So if this is indeed a piezo-equipped guitar:
If the guitar doesn't have its own preamp, you will need a suitable one inbetween the guitar and the Focusrite.
If the guitar does have its own preamp (like a Fishman or similar) it is usually battery powered and has controls for volume/gain and a basic or more advanced tone control. Check the battery and the controls. In such configuration, the Focusrite needs to be switched to "instrument level" on the input you use.
Sometimes (but more rarely), these built-in preamps are and/or can be powered by phantom power, in which case things may get a little more complicated in making the proper connection. And the Focusrite should then be able to deliver phantom power, which I don't know whether it does.
Hope this gives some more clues to get things going.
Cheers and happy connections,