You will get three seats in the back but not using the intregrated seats I don't think.
We had a v50 which is only a bit smaller with the intergrated seats and you could not get a standard car seat in between the two boosters. You might just squeeze in a fairly slim stage 0 seat but I wouldn't count on it.
Have you a local volvo show room to visit and try the seats in?
I think a four year old should be ok in a booster seat, my 4 year old ds uses one, but i'm sure some people would strongly disagree on that.
Our v50 was lovely (fond memories of bum warmers in the seats on cold winter days!)and volvo make fantastic cars so go for it!
thanks for the reply. i think we will just go for it. can't bear the thought of a people carrier just yet. that was what i thought will go and try the seats we have and won't rely on the integral boosters.