Assistance with nested formula

Can someone assist?  I have created a formula which is working to a point.  I just can't figure out how to stop it providing information if the cell has no data in it.

What I need is the No of hours  Column M to reflect "0" if there is no time inputted into column N.  I am getting the correct result for the formula when a time is inputted. I adjusted the time to make the checks.

Could anyone offer any advice? If I have used the wrong formula let me know. I am new to the IF statements.

Formula: =IF(N4<=$S$4,"1",IF(N4<$S$5,"2",IF(N4<$S$6,"3",IF(N4<$S$7,"4","0"))))   

 =IF(N4<=$S$4,"1",IF(N4<$S$5,"2",IF(N4<$S$6,"3",IF(N4<$S$7,"4","0"))))