I look at this differently than what appears to be the perspective of most...

I understand the idea of "room recording" of a quartet and the idea of concert hall reverberation... but I would not use them (the quartet) to create quartet concert pieces (I suppose some would)... I would use them in the context of a song/lyric, typically a Country or Pop ballad and the suggested tight panning crowds the vocal. The only thing I put at 15% is a tight, duet harmony (vocal) or a featured solo instrument.

I often create my own version of the quartet using the individual tracks (often using a background fiddle as one or both of the violins) to get more control over the tracks and their panning... which brings up an aside - if the individual tracks will always be provided in addition to the quartet as they were with the first one that would be great!

My panning generally ends up more like:
violin1 40-45%R (I like that one on the right)
cello 45-55%L (and the cello on the left)
viola 30%R
violin2 30%L

floyd