Remember that
The compound interest formula is equal to
where
A is the Final Investment Value
P is the Principal amount of money to be invested
r is the rate of interest in decimal
t is Number of Time Periods
n is the number of times interest is compounded per year
in this problem we have
P=$ 4,000
A=$7,148
r=9.4%=0.094
n=4
substitute the given values
apply log both sides
t=6.25 years