BUG ON FUNCTION IF ?

Hi, I have an issue on Excel, I think is a Bug. So follow this simple test:

Always Format Hour for All cell

1) Into Cell B4 put 08:00:00  

2) Into Cell C4 put 12:00:00

3) Into Cell F4 put formula =IF(SUM((C4-B4) + (E4-D4))=0;"";SUM((C4-B4) + (E4-D4)))  -> ( Result 04:00:00 )

4) Into Cell G4 put 04:00:00

Now.... Into Cell H4,

if you put G4-F4 -> ( Result 00:00:00 )

BUT if you put  =IF(F4="";"";G4-F4) I give an error -> ( Result ################### )

Why ??? It's a bug ??? 

Thanks !