I don't think a weaker spring will help it cycle faster. You need a stronger spring or springs. The pressure in the chamber will help the valve close, but the spring is also needed. At 4000 rpm the firing cycle must be completed in 15 ms! I do not have the exact spec on the Drozd solenoid, but on ones I think are similar, you will have to have high power and limited travel to be able to cycle the solenoid that quickly. The same applies to the valve. In that 15ms time you have to accelerate all the moving parts, do some work, accelerate in the opposite direction, get back to the closed valve position and have the solenoid back in a reset position.
Stiffer springs will make the return part of the cycle faster with less of a slowdown on the strike portion.
TBK