4 taps fill 100 litres in 6 hours, so the rate of one tap is:
\[ \text{Rate of 1 tap} = \frac{100}{4 \times 6} = \frac{100}{24} = \frac{25}{6} \text{ litres per hour}. \]
Since 8 taps will fill at double the rate of 4 taps, the rate of 8 taps is:
\[ \text{Rate of 8 taps} = 8 \times \frac{25}{6} = \frac{200}{6} \text{ litres per hour}. \]
\[ \text{Time} = \frac{150}{\frac{200}{6}} = \frac{150 \times 6}{200} = 4.5 \text{ hours}. \]
The time required to fill the tank is 4.5 hours.