Cx. A double sharpened C Enharmonically the same as a D, but you'll have to call it Cx in this context
Well, when you look at any (mode of a) major scale, the basic idea behind is that every note should be there, with added flats/sharps as needed, so that no note is there twice. For example, the key of C: C D E F G A B C. You don't call the B a Cb because there's already a C!

The guy asked for an augmented fourth in G#, the G# scale is G# A# B# C# D# E# Fx G#.
You'll see the fourth, which is a C#. To make an augmented fourth out of the natural fourth, we have to sharp it and since that has already happened it becomes a double sharp!
stuff calling it differently, i know youre ment to call an augemented 4th C## or something, but it IS the same as D
:: by the way you know any easy fingerings for aug or diminished chords, or diminished or aug scales?
- tommy