Question

How many capacitor of 4uF capacity each are combined to get total capacity of 6uF

Answer 1

You could do this if you use three 4uF capacitors in the following arrangement:

C1 and C2 in series, and then the equivalent arrangement of C1 and C2 in parallel with C3.

Calculate the equivalent capacitances and see why that works.

Answer 2

We start by connecting three 4uF capacitors in parallel which gives us a 12uF capacitor. Now if we connect three more 4uF capacitors in parallel, we'll have two 12uF capacitors. Now if we connect these two 12uF capacitors in series, we'll have exactly 6uF:

C = C1*C2/(C1+C2).
C = 12*12/(12+12) = 6uF.