VBA - Import Macro - How do i add to the current data when importing?

Question

I have created a macro to import data from another workbook, but i have to set which row to start. I was wondering how i would alter the code to paste the import data to the last empty row (so its adding to the table).

Currently the macro, prompts to click the file you would like to import, then imports the data from certain col from source WB to target WB. But as you can see it as been assigned the row to paste. My question is how do i get it to paste at the last empty row, so it is collecting the data rather then over writing the data.

Would i need to change the "targetSheet.Range("R2", "R4000").Value = sourceSheet.Range("Q2", "Q4000").Value"

Public Sub Extract_Excel_file()

''//--------------------------------------------
' Get customer workbook...
Dim customerBook As Workbook
Dim filter As String
Dim caption As String
Dim customerFilename As String
Dim customerWorkbook As Workbook
Dim targetWorkbook As Workbook

' make weak assumption that active workbook is the target
Set targetWorkbook = Application.ActiveWorkbook

' get the customer workbook
filter = "Text files (*.xlsx),*.xlsx"
caption = "Please Select an input file "
customerFilename = Application.GetOpenFilename(filter, , caption)

Set customerWorkbook = Application.Workbooks.Open(customerFilename)

' assume range is A1 - C10 in sheet1
' copy data from customer to target workbook
Dim targetSheet As Worksheet
Set targetSheet = ThisWorkbook.Sheets("Raw Data")
Dim sourceSheet As Worksheet
Set sourceSheet = customerWorkbook.Worksheets(1)
sourceSheet.Range("A2", "V400").NumberFormat = "@" ''//Set all cells to 
text format.

' sourceSheet.Range("A2", "A4000").NumberFormat = "@" ''//Set all cells 
to text format.
targetSheet.Range("B2", "B4000").Value = sourceSheet.Range("A2", 
"A4000").Value
targetSheet.Range("C2", "C4000").Value = sourceSheet.Range("B2", 
"B4000").Value
targetSheet.Range("D2", "D4000").Value = sourceSheet.Range("C2", 
"C4000").Value
targetSheet.Range("E2", "E4000").Value = sourceSheet.Range("D2", 
"D4000").Value
targetSheet.Range("F2", "F4000").Value = sourceSheet.Range("E2", 
"E4000").Value
targetSheet.Range("G2", "G4000").Value = sourceSheet.Range("F2", 
"F4000").Value
targetSheet.Range("H2", "H4000").Value = sourceSheet.Range("G2", 
"G4000").Value
targetSheet.Range("I2", "I4000").Value = sourceSheet.Range("H2", 
"H4000").Value
targetSheet.Range("J2", "J4000").Value = sourceSheet.Range("I2", 
"I4000").Value
targetSheet.Range("K2", "K4000").Value = sourceSheet.Range("J2", 
"J4000").Value
targetSheet.Range("L2", "L4000").Value = sourceSheet.Range("K2", 
"K4000").Value
targetSheet.Range("M2", "M4000").Value = sourceSheet.Range("L2", 
"L4000").Value
targetSheet.Range("N2", "N4000").Value = sourceSheet.Range("M2", 
"M4000").Value
targetSheet.Range("O2", "O4000").Value = sourceSheet.Range("N2", 
"N4000").Value
targetSheet.Range("P2", "P4000").Value = sourceSheet.Range("O2", 
"O4000").Value
targetSheet.Range("L2", "L4000").Value = sourceSheet.Range("P2", 
"P4000").Value
targetSheet.Range("Q2", "Q4000").Value = sourceSheet.Range("L2", 
"L4000").Value
targetSheet.Range("R2", "R4000").Value = sourceSheet.Range("Q2", 
"Q4000").Value



' Close customer workbook
Application.DisplayAlerts = False ''//Don't promt to Save
customerWorkbook.Close
Application.DisplayAlerts = True   '' undo Don't promt to Save

End Sub

I just want it to collect the data rather then overriding it each month.