Excel Macro copy to cell based on criteria

hey everyone,

I'm stuck. I hope someone can help me out today. I have an excel spreadsheet with 1400 rows. The first 3 rows are irrelevant (headings). I have 3 cells with $tring content (B:C:D). What I need to do is create a macro that can specifically look in column B and column D for non-null values. There are multiple cells in this spreadsheet that must have the same value. Column B and D must be compared, and must be found with non-null values. And, in the same instance where Column B is also found again (loop) (same cell content, but different row) Col D will have a null value (missing content). I need to fill this Col D with the same content as the first record that had Col B and D content. In my example the value is hard coded, I don't know how else to do it. I'm very lost and I need some help to complete this. This is what I have.

Sub _MacroUser10()

Set I = Worksheets("Sheet2")
Dim d
Dim j

d = 1 'loop counter
j = 4 'start at the 4th row because of headings

Do Until ActiveCell(I.Range("B" & j) = "User Portfolio Group") And ActiveCell(I.Range("D" & j <> null)) 'loop

If I.Range("B" & j) = "User2010" Then 'the cell is entitled User2010...etc
If I.Range("D" & j) = " " Then 'this is supposed to find the null values in D
d = d + 1
End If
i.Rows(d).Value = I.Rows(j).Value
End If
j = j + 1
Loop

End Sub

All in all. I need to lookup the content in Cell B$ and lookup the content in Cell D$. There will be one row with content in both places. Then I need to find all the other matching rows that match B$ that do not have any values in D$. I need to add those missing values from the non-null D$ to the null D$. I hope that actually makes sense to someone :(

I hope someone can help :) :) :)