Had the same issue on both the 08 +4's II have owned/own. Finally tracked down that the MMC had issues with rear springs around this era and have changed suppliers in recent years, basically the springs from around 06-2010 were too soft, this is the case on 2 and 4 seat cars (I have had both types).
There is a very good fix to fit new rear springs, however, after talking it through with Peter Mulberry and him advising me that several owners had him supply 4 seat srings for a 2 seat car (this was done as they normally drive 2 up and often carry luggage) I went this route and never looked back on my current 2 seat car.
I worried it may be too bouncy or harsh but it's not at all, initially the ride height was raised to about 15mm above a standard brand new car measured at a dealership, and this has settled and dropped 10mm from that over the first few months where it has stayed ever since. I run Koni Classic adjustables on the lowest setting on the rear and 2 clicks up from softest on the front.
