Well, the conditions and cell references are different than your original question, as you had stated that you wanted it to return "N/A" for values less than or equal to zero, and your new equation only returns "N/A" if it is exactly equal to 0.
If you new formula, accomplishes what you are really after, then as long as you only have numeric values in H4, you don't need a nested IF. You can simplify your formula to:
Code:
[COLOR=#333333]=IF(H4<>0,H4/F4,"N/A")[/COLOR]
because if you only have numeric values in H4, then if you check to see if the value is NOT zero, the only other option is that it is zero (so you don't really need to check for that value, you can just say return "N/A" otherwise).