This is all probably quite wrong, but it's something to think about...

Something has been bothering me a little about the solution to G4. It seems that Chelsea has confirmed that glyph #4 solve to 'T'... but something about the solution seems unsettling; it's a very obvious, one-step solution that doesn't use numbers in any way, shape or form.

However, consider this:

A = 1
C = 3
G = 7

Yes? Now, if we suppose that the sequence is that of odd numbers, we have:

A = 1
C = 3
? = 5
G = 7

What fills in the '?'? My only theory involves returning to genetic bases. The four commonly-known amino acids that form the base pairs of DNA are abbreviated A, C, G, T. There is, however, a "fifth" base found in RNA, which carries the abbreviation U (if I remember my biology correctly, it bonds with A in place of T).

So, could the fourth glyph in fact be, "U"?