Excel formula for calculating a balance

Hi. I am putting together a relatively simple spreadsheet to keep track of income and expenditure. 

I have a starting balance in cell H3.

I want to put a formula in cell H4.

If an Income value >0 is input into Cell E4 I want it to add to the H3 value and calculate the answer in H4. OR if an expenditure value >0 is input into F4, I want it to subtract F4 from H3 and calculate into H4. There will never be a value >0 in both E4 and F4. Any help would be gratefully received. Thanks.