Simple Formula-Add number to existing cell

I am new to this forum and fairly new to Excel 2003. My question is I need a simple formula that when I enter a numerical value in a cell it will automatically add 4 to it and display the results. One of the cells is C6. I have several cells that the formula will need to be in. I also want the formula not to show in a cell when no value is entered. My second question is how do I protect certain cells from being altered and the other cells can be altered? Just call me Newb! Thanks for all help!