Oh, French units then. I can do that.
So 4 springs with a rate of 4000 N/m adds up to 16000 N/m and will yield a natural Frequency of 3.3 Hz. A little bit lower spring rate will lower the frequency some more.
Some spring mass system calculators can be found on the web to verify the calculations are done right.
I’m an old Aerospace engineer but I also worked in Automotive so I can operate in both systems of measurements although I think mostly in English units.