Which of the following pairs of fractions are not equivalent?

1. $\dfrac{3}{4}, \dfrac{15}{20}$

2. $\dfrac{14}{21}, \dfrac{4}{6}$

3. $\dfrac{8}{10}, \dfrac{12}{15}$

4. $\dfrac{6}{14}, \dfrac{10}{25}$

Check each option using cross multiplication:

1. 3 × 20 = 60 and 4 × 15 = 60. Equal, so equivalent.

2. 14 × 6 = 84 and 21 × 4 = 84. Equal, so equivalent.

3. 8 × 15 = 120 and 10 × 12 = 120. Equal, so equivalent.

4. 6 × 25 = 150 and 14 × 10 = 140. Not equal, so not equivalent.

Hence, option 4 is the correct option.