Add New row, keep same formula in column

Hello!

I have a question about keeping a formula in a column and having it duplicate in that column each time a new row is inserted. The formula I have right now is..

=IF(G4="3 Phase SD",D4+5,IF(G4="Corporate SD",D4+5,D4+3))

I tried using...

=arrayformula(IF(G4="3 Phase SD",D4+5,IF(G4="Corporate SD",D4+5,D4+3)))

I inserted a row below it and it didnt work. Any ideas? The idea would be the following, insert 1 row below and it populates...

=IF(G5="3 Phase SD",D5+5,IF(G5="Corporate SD",D5+5,D5+3)) and so on and so forth.

Thanks in advance