Without any more information, it sounds like interest is compounded continuously, in which case the value of the investment <em>A</em> is given by

where <em>P</em> is the principal investment, <em>r</em> is the interest rate, and <em>t</em> is the number of years.
At the end of the second year (<em>t</em> = 2), the value is <em>A</em> = 60,500, and after the fourth year (<em>t</em> = 4), the value is <em>A</em> = 73,205. So solve the system

for <em>r</em>. You can eliminate <em>P</em> by dividing

Take the logarithm (log here means natural log) of both sides to get
2<em>r</em> = log(121/100)
<em>r</em> = 1/2 log(121/100)
<em>r</em> ≈ 0.0953
So the interest rate is about 9.53%.