Excel vba count last row source file - vba

Question

I am relatively new to VBA.
I have a target workbook where the first step is somebody has to write something. If not an error message pops up.
After they filled in their data the code will ask them to open the source workbook(they downloaded the file before). From the source workbook certain columns are read. This all works fine in the code. The problem I have is to get what they filled in the target workbook to be copied down in column (A4:A(last row in source workbook). So the length until where their manually entered data has to be copied down has to be equal to the length of data in the source workbook.
Sub get_rate_codes()
Dim CheckCell As Range
Dim wb_source As Workbook
Dim wb_target As Workbook
Dim strPathName As String
Dim lastRow As Long
For Each CheckCell In Sheets("rate_codes").Range("F3").Cells
If Len(Trim(CheckCell.Value)) = 0 Then
CheckCell.Select
MsgBox "Cell " & CheckCell.Address(0, 0) & " is empty. Please enter SITA."
Exit Sub
End If
Next CheckCell
'start to open file
Application.ScreenUpdating = False
'start is the starting cell while lastRow measures the last data row in the external file
Start = 4
'continue to copy data from the rate codes report
Set wb_target = ActiveWorkbook
With wb_target.Sheets("rate_codes")
lastRow = wb_source.UsedRange.SpecialCells(xlCellTypeLastCell).Row
strPathName = Application.GetOpenFilename()
If strPathName = "False" Then
Exit Sub
End If
Set wb_source = Workbooks.Open(strPathName, 0)
.Range("B" & Start & ":B1000").Value = wb_source.Sheets(1).Range("E2:E1000").Value
.Range("C" & Start & ":C1000").Value = wb_source.Sheets(1).Range("H2:H1000").Value
.Range("D" & Start & ":D1000").Value = wb_source.Sheets(1).Range("G2:G1000").Value
.Range("E" & Start & ":E1000").Value = wb_source.Sheets(1).Range("K2:K1000").Value
.Range("A" & Start & ":A" & lastRow).Value = wb_target.Sheets(2).Range("F2").Value '
wb_source.Close (False)
End With
'close file without saving
Application.ScreenUpdating = True
End Sub

Answer 1

To find the last row you can do this.
Dim lastRow As Long
lastRow = wb_source.Cells(wb_source.Rows.count, "A").End(xlUp).Row
Then you can write to the next line.
.Range("A" & lastRow + 1).Value =