i have 4000L in 6 hours. what is the cubic meters per second?

What I always remember is that 1 cm cubed = 1 mL

$$\boxed{1cm^3=1mL}$$

$$4000L=4000000ml=4000000cm^3$$

Now

$$100cm=1m\\ 100^3cm^3=1m^3\\ 1000000cm^3=1m^3$$

so

$$4000000cm^3=4m^3$$

$$4m^3\quad\mbox{of water in 6 hours}\\\\ \frac{4m^3}{6\times60\times 60\:\:seconds}\\\\ \frac{1m^3}{6\times15\times 60\:\:seconds}\\\\ \frac{1m^3}{5400seconds}\\\\ \approx 1.85\times 10^{-4}\quad m^3/sec$$

(4000L / 6 hr) x (1 cubic m / 1000L) x (1 hr / 3600 sec)  = (1/5400)m³/sec = 0.000185m³/sec