Time of investment is 3 years.

Amount invested is four times the interest earned in 1^st 3 years.

& principle amount = Rs. 25000

∴ Principle amount = 4 × (Interest earned in 1^st 3 years)

Simple interest = $(\frac{{\left( {principle\;amount\; \times \;rate\;of\;interest\; \times \;time} \right)}}{{100}})$

$(\therefore 25000 = \frac{{4 \times 25000 \times rate\;of\;interest \times 3}}{{100}})$

∴ 100 × 25000 = 12 × 25000 × Rate of interest

∴ Rate of interest = 100/12

∴ To find interest earned at the end of 4 years -

Simple interst = $(\frac{{principle\;amount\; \times \;rate\;of\;interst\; \times \;time\;of\;investment}}{{100}})$

∴ Simple interst = $(\frac{{25000\; \times \;100\; \times \;4}}{{100\; \times \;12}})$

∴ Simple interest earned in 4 years = Rs. 8333.33

∴ At the end of 4 years Sunil will earn Rs. 8333.33 interest.

Alternate:

As we know, Amount invested is four times the interest earned in 1^st 3 years

Let ‘I’ be interest for 1 year

According to the question;

⇒ 25000 = 4 × 3 × I

⇒ I = 25000/12 = 2083.33