a shot in the dark:
Sub corpsman000()
Dim SRow As Long, LRow As Long, DRow As Long, CLetter, i As Long
i = 20
CLetter = ActiveCell.Value
Columns(4).ClearContents
LRow = Cells(Rows.Count, 3).End(xlUp).Row
'If DRow = Cells(Cells(Rows.Count, 4).End(xlUp).Row, 4).Row < 20 Then
' DRow = 20
'Else
' DRow = Cells(Cells(Rows.Count, 4).End(xlUp).Row, 4).Row
'End If
DRow = Application.Max(20, Cells(Cells(Rows.Count, 4).End(xlUp).Row, 4).Row)
Do Until i = LRow
If Cells(i, 3).Value = CLetter Then
Cells(DRow, 4) = Cells(i + 1, 3).Value
DRow = DRow + 1
End If
i = i + 1
Loop
End Sub