• Citroen C4 Picasso Front Axle Parts
• Citroen C4 Picasso Cargo Space

Citroen will sell you one. The seat base cushion is listed in the parts and includes its cover. To get the right part no. Needs your VIN but a quick check indicates a new one may be around £700. On our first C4 (2008) and C3 (2003) bits of foam from the seat would appear on the floor next to the door and the seat base cushion at the door edge seemed to be losing its padding.

Citroen C4 Picasso Front Axle Parts

Citroen C4 Picasso Cargo Space

I would imagine that if a large person used the car then the seat bases would not last long. I also remember posts on the C5 seat frames breaking.