Go into the atribute editor for particleShape.
Under Lifespan section, select [LifespanPP only].
Scroll down to Per Particle (Array) Attributes, hold right click on lifespanPP column and Create Expression.
Add a creation expression to lifespanPP that determines the lifespan per particle. Use the function rand() as a random value generator to generate a random lifespan between 4 and 6 (seconds).
lifespanPP = rand(4,6);