The only cell that formula will ever look at is C4. Using the $ symbol before a column/row in a formula like that makes it an absolute reference - i.e. look here, always here; no matter where you copy and paste this formula to, it will always look at the value in C4.
Remove all of the $ symbols from your formula, and it becomes a relative reference. it will look at whatever the value is in column C of the same row that the formula is in.
Absolute references are good for things that are always the same, no matter what, say, sales tax. Relative references are when you have a value that is specific to that individual record/row, like quantity.
