You might want to read the last few posts here:
http://www.dakkadakka.com/dakkaforum/posts/list/60/600659.page I think the conclusion (too long ago to be in the debate) for me was this:
a) A vehicle is in ruins and its facing is at least 25% obscured => 4+ (p.77/p.108)
b) A vehicle is in ruins and its facing is not or les than 25% obscured => No Save (Second bullet point on p77)
c) A vehicle is not in ruins, but part of ruins obscure its facing by at least 25% => 4+ (p.77)
C) is the point of contention, but please note p77:
1st bullet point:
"25% (...) needs to be hidden by intervening terrain or models"
I am quite sure i know what "intervening" means, and you cannot be "IN" models.
4th bullet point:
"exactly like a non-vehicle model would do against a wound"
non-vehicle models taking a wound are exactly the same value: 4+