I am not sure about this but the exclusive might add sat nav, reversing sensors front and rear, chrome bits on the body, larger alloy wheels, climate control with separate left/right temperature control, possibly a glass roof (panoramic) but this might be an option, different seat cover material, the suspension might have air bags to make it softer/self levelling but I would query this since they can fail, it might have an alarm.
If you have a look on the Citroen Picasso forum you ought to get a better reply than this. If you have not already joined that forum it is worth doing. http://www.citroenpi...org.uk/picasso/
You can have a look at the handbooks on the service.citroen site but it will not give the difference between the models since they just list everything without saying which applies to which model. http://service.citroen.com/ACddb/ you might need to select your language first. The C4 Picasso latest models are now called a spacetourer.
If it is a petrol engine you are going for make sure the timing chain has been replaced since these do not last long. We had a 1.4 vti petrol C3 and at 50k miles it needed doing. It also used a lot of oil from when we got it at 12k miles, about 1 litre every 1500 miles.So I would make sure you get a good warranty.
If it is a 1.6 hdi diesel engine check the turbo is not worn and its service history for annual oil/filter changes using the correct oil. Make sure the exhaust dpf is still in place, the exhaust tailpipe should be clean internally, if not then the dpf could have been removed/knocked through.