Keith has received a rate of return of 37.5% over the four years.
To calculate the rate of return, we can use the formula:
Rate of Return = ((Final Value - Initial Value) / Initial Value) * 100%
Given that Keith's initial investment was $4,000 and it grew to $5,500 over four years, the rate of return can be calculated as:
Rate of Return = ((5,500 - 4,000) / 4,000) * 100% = 37.5%
Therefore, Keith has received a rate of return of 37.5% over the four years.