To find out how many minutes Rajani would practice in 4 weeks, we first need to determine her weekly practice time.
Given that she practices 840 minutes in 3 weeks, we can calculate her weekly practice time as follows:
\[ \text{Weekly practice time} = \frac{840 \text{ minutes}}{3 \text{ weeks}} = 280 \text{ minutes per week} \]
Now, to find out how many minutes she would practice in 4 weeks, we multiply the weekly practice time by 4:
\[ \text{Total practice in 4 weeks} = 280 \text{ minutes/week} \times 4 \text{ weeks} = 1120 \text{ minutes} \]
Thus, Rajani would practice a total of \( \boxed{1120} \) minutes in 4 weeks.