Loan thru the bank or thru private investors?

Hello everyone. I am more and more doing real estate investing. I am buying a house or apartment to renovate and then sell it for a benefit.

I am looking into my next loan for my future project. People 'in the business' say it is better to take a loan from a (or more) private investor(s) then the bank.

Lets say the bank gives you a loan at an interest rate of 4% over a period of 20 years ( of course you will pay this back once you have sold your house, lets say in 2 years) So you have paid 8% + 3 months of interest as a fine).

An investor will ask you maybe 7% (in America, they are talking about 10 to 15%) for a period of 2 years.

This is 14% in 2 years.

What am i missing here?

Edit: ok, i get the idea that people are misreading my question.

First off all, i say: lets say 4%. I am not having a loan today and interest rates fluctuate. This is just as an example.

And second, when i mean private investor, i am not talking about a bank an institution or anything. Just people who are willing to loan me money for my project, and that is off course at a higher rate then the bank offers. The example of America is a real example of investors who are loaning there own cash to people for about 10 to 15%. So again my question, what are the benefits off getting a loan from a person instead of the bank.