Multiply Part of a Number by Different Rates Depending on the Size of the Number

Hi All

I need a little help please.

I want a single formula that will allow me to multiply a number by different values depending on how big that number is.

For example:

Say the number is 7.

I want to multiply 4 x a specific rate and then the remainder in this instance 3 by another rate.

Say the number is 12

I want to multiply 4 x a specific rate and then the remainder in this instance 8 by another rate.

That looks perfect thanks a million for your help!