It is my understanding that B4s are 15-20% firmer than OE spec but retain the same (or better in Alfas case) ride quality.
B4 should be in theory a little better than factory ones as they are gas presurized monotube damper but their dampening is set to factory setting. They are still in Bilsteins line of "spare parts" and not "performance dampers". Actually there are still B2 dampers in the lower line that are only oil filled damper with the same construction as OEM. B4 are oil/gas pressurized so there is difference in construction, but should have the same characteristics.
It is similar as B6 and B8. Their dampening is exactly the same, B8 have just 30mm shorter piston for lowered cars.
So the ride on B2 and B4 should be exactly the same as on OEM even though they have different construction (B4 being superior). Benefits of B4 being monotube is larger piston diameter, better heat dissipation and less oil foaming due to being under gas pressure. They may last longer and the dampening will not change so much depending on temperature and load, but they will still not be stiffer.
Of course, if you replace few years old worn dampers with new ones, even with the same type, the new ones will feel better, stiffer and better controlled. So I am sure if you go for B4, they will feel as a sport suspension compared with the worn old ones
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