Glass mentioned in that Red Bull Music Academy video that he was at
work on a string quartet.  The premiere of this work by the Kronos
Quartet, his first string quartet since 1991, will take place in Vancouver on Oct.19th.

This is of great interest because throughout the Glass catalogue,
there seems to be a consistency of output among types of work.  For
example, operas are a consistent part of his catalogue; never does too
much time elapse between operas.  Since he began writing traditional
concert works circa 1987 with the first Violin Concerto, concertos and symphonies have been a regular part of his output.

One could probably view the neglected part as being chamber music during that period.  Since 2007 with Songs & Poems (No.1)
Glass re-invested himself in small scale composition.  There is a
caveat to this, that though Glass wasn't writing concert pieces for
chamber ensemble he was writing for chamber groups for the theater in
pieces like The Sound of a Voice (2003)(flute, cello, percussion, and pipa), Dracula (1999) (for string quartet) and In the Penal Colony (2000) (string quintet).  So after 2007 we start to see a real interest revived in this type of work with his first Violin Sonata (2008), Two Movements for Four Pianos (2008), Songs & Poems No.2 (2010), Partita for Solo Violin (2010-11), Orbit for Solo Cello (2013), Pendulum for Violin & Piano(2010), Duos Nos.1-5 (2010) and Two Movements for Four Pianos (2013).

It must be said that string quartets are the most intimidating form
for most composers. In the notes to the Kronos recording of Glass
quartets, Mark Swed discusses the composition of quartets with Glass and
about how the medium has always been a sort of mirror for composers to
hold up to themselves – to see their own qualities and faults.  It's
viewed to be the most serious medium and often is.  So the trajectory
and linear progressive quality of Glass' quartets is fascinating to me.

The first quartet from 1966 is very early
Glass. He had not yet developed his musical language to the  point of
fluency that he found within 3 or 4 years. But it is repetitive. Its
architecture is basic but forward-looking. String Quartet No.2
is comprised of sections of music taken from the Glass/Akalaitis
production of Beckett's play Company in 1980 and is an effective (and
very popular) miniature at only 8 minutes which showcases Glass at his
most concise.  String Quartet No.3 is
comprised of sections of music taken from Glass'  score to the film
Mishima.  At 15 minutes and written in 1985, that piece shows Glass
really investing himself for the first time in the medium.  So really
with String Quartet No.4 "Buczak" in the late
1980's does Glass start his first "absolute" string quartet.  No.4 is a
remarkable accomplishment topped only by String Quartet No.5.

With this linear progression in mind, that each quartet is longer and digs deeper, we look forward to this fall's premiere of String Quartet No.6. 
Evidence that Glass is taking a deep breath before this piece is that
his quartet music for the film Bent (1997), when it was finally recorded
and released on the Brooklyn Rider album, did NOT receive a number
designation like Mishima had.