There are a few ways to do this, and if your formula isn't firing a value, it's either an issue with a mismatch of value types, (ex time vs simple numeric) or an issue with your formula syntax Without seeing the field properties for 1-3, it's hard to diagnose the issue with the formula, but the end result your after is a duration type formula field.
I agree with Dan however, The only use I can see for the 4th field is to "convert" the unit of measure to the duration type you want, when it might be easier to change the format of fields 1 and 2 to return a duration value instead, which would make Dan's formula correct.
Hope this helps!