If Cell Is Blank, Fill With Other Cell’s Text Data

This is probably simple for you experts out there. I've tried so many ways, and in so many locations w/in the code, but none are working for me. Please help!

All I’m trying to do is get data from CELL RANGE B4:B4000 to copy automatically to CELL RANGE J4:J4000 should CELL RANGE J4:J4000 be empty.

FYI: Data being inputted via a user-form.

Option Explicit

Private Sub CmdButton_CONTINUE1_Click() 

Dim TargetRow As Integer
Dim FullName As String   'Variable for FULL NAME = CELL RANGE J4:J4000
Dim QBFileName As String   'Variable Quick Books File Name = CELL RANGE B4:B4000
Dim UserMessage As String

FullName = Txt_Client_First_Name & " " & Txt_Client_LAST_Name
QBFileName = Txt_QB_File_Name

'begin check if EDIT or ADD New Entry Mode
If Sheets("Engine").Range("B4").Value = "NEW" Then  'ADD New Entry Mode


'BEGINS VALIDATION CHECK: IF in "ADD New Entry Mode" mode to prevent duplicate FULL NAME J Column entries

If Application.WorksheetFunction.CountIf(Sheets("Database").Range("J3:J4000"), FullName) > 0 Then

MsgBox "Client's Full Name already exists", 0, "Check" 
Exit Sub 

End If  'ends validation check OF Duplicate FULLNAME (J Column)


'BEGINS VALIDATION CHECK: IF in "ADD New Entry Mode" to prevent duplicate QBFileName B Column entries

If Application.WorksheetFunction.CountIf(Sheets("Database").Range("B3:B4000"), QBFileName) > 0 Then

MsgBox "QuickBooks File Name already exists", 0, "Check" 
Exit Sub 

End If
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