Hmmm..... I'm a bit
McPherson suspension doesn't give a lot of wheel travel to start with.. then lowering by 4cm reduces that even more... so it's a tricky one to get right. You could say even Alfa didn't get it perfect yet..
The temptation for lowering spring manufacturers is to make the springs a lot harder, so that the car doesn't run out of suspension movement and hit its bump stops. Too-hard springs can make the car "jump" over bumps and is generally more uncomfortable the rest of the time.
Progressive springs are good... but if the intial part of the spring is too soft and/or the later part of the spring is too hard, then you end up with just the "progressive" part of the spring doing most of the work.
I'd guess too that the springs are designed to work with a modified (re-valved) damper.. since a normal damper compressed by 4cm thinks the springs are under compression, even when they're not. It will behave differently to a fully extended damper, even with the same load (since dampers act differently at different levels of compression).
I'd guess the damper is trying to damp what it thinks is quite a big shock, while the springs are progressive and are dealing with small compressions... so there's a mismatch between what the spring is doing (trying to extend the damper) and the damper (trying to damp what it thinks is a big shock).
If you want to keep a standard damper (easiest to find?) then a <30cm lowering spring would be better.. If you can find an Eibach set then that might be the cheapest way to go (since dampers are always more expensive).