It might be a bit late by now, but the simple solution is to take out the air bag springs and replace with metal springs. I hava a 08 C4GP, computer wouldn't tell the pump to turn on, so bags wouldn't fill, when they did eventually fill, car was drivable, until you put any weight in the boot or people in the rear, it would release the air and hey presto we were low ridin, Califonian style.

Solution - bought 2 used Metal springs from a breaked picassco, bought new rubber bump stops from Citroen(they go on top of metal spring), bought spring compressor clamps. Jacked car up on to stands, the rubber bags lift out in your hand, compressed metal spring and fitted it, repeat on opposite side. Then remove outer cover off air pump located on underside of boot floor, remove inner cover, disconnect power cable. Then cut both air hoses at each bag, rolled them up and placed inside, inner cover, while putting back on, put outter cover back on. Drop car back to ground, presto... A fully working, non irritating, C4 Picasso.

Note the "Suspension Faulty" warning and Service light on dash will still come on, here in Ireland that is a Test fail, so begrudgingly took it to Citroen to have mechanic hook it up to his laptop and turn it off, cost €50.

Metal springs €100

Bump stops €40

Coil spring compressors €40

Service light turn off €50

Total €230 and it is sorted for an age, note that the bump stops do perish and require replacement ever 5 years or so, depending on individual driving conditions etc...so the clamps will be used again.

Regards your problems with C4 rear suspension,mine has been playing up on my 59 plate exclusive and main agent confirmed problem and said Citroen couldn't help as I didn't have full main agent service history and quoted little under £500. Bought the bits on eBay for 90 pounds and friend fitted them for 30pounds, if you can get the rear on stands you don't even have to take the wheels off and takes half hour . The bits are pattern but quality , problem sorted for 120quid,joe