For 4% interest rate or APR, and w/ 10K principal, lets say you put it in the bank for a year.  Then, you will get:

$10,000*(1+4%)=$10,400

Annual Percentage Rate, or APR, is a measure of how much interest will be on an annual basis without taking into account compound interest. Annual Percentage Yield, or APY, is the same measure of interest rate, but accounting for compound interest - a better measure of how much you will pay in interest. Banks and credit card issuers often express credit card interest rates in APR, in order to better hide just how much interest would cost.

Next case, if you have 5% APY, we will have to convert it to APR.  The formula is simple like this:

APR = Period Rate x Periods per Year

Therefore:

5%=((1+Period Rate)^12)-1 ==>

1.05=(1+Period Rate)^12 ==> Period Rate = .004074

Recalling APR = Period Rate x Periods (in this case, 12), then the APR is 4.89%

So, you are not getting what you might think you were based on an ads.  But 5% APY is still better than 4% APR, since you are comparing a 4.89% APR to a 4% APR (apples to apples).

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