VBA code to Filter data automatically by windows log-in user ID?

Since you have a small number of users, you can hard code this directly in your VBA project. To do this I will use the Dictionary object. A dictionary is a way to store data in a key, value pair.

Let's dive into some code.

Sub Test()

    Dim dict As Object
    Dim key As Variant
    Dim user As String
    Dim region As String

    Set dict = CreateObject("Scripting.Dictionary")

    'Add your username / region
    'pairs to the dictionary object
    dict.Add "user1", "South"
    dict.Add "user2", "North"
    dict.Add "user3", "South"
    dict.Add "user4", "West"
    '.. etc

    'Get the username of the currently logged-in person
    user = Environ("username")

    'Loop through dictionary to find the
    'region which matches the username
    For Each key In dict.Keys
        If key = user Then
            region = dict.Item(key)
        End If
    Next key

    'If the username is not found, we should
    'exit the subroutine.  You could display
    'a messagebox or something similar
    If region = vbNullString Then
        MsgBox "Invalid username!", vbCritical, "Invalid Username"
        Exit Sub
    End If

    'From hereon out you would do as you
    'normally do, passing in the variable
    'region as one of your parameters


End Sub

If you don't want users to see which regions other users are assigned to you should password protect your VBA code. You can do this by going to Tools --> VBAProject Properties... --> Protection and tick the 'Lock project for viewing' box and enter a password.

I should add that using this dictionary method has limitations. If one user needs to be assigned to multiple regions, for example, this will not work as each key, value pair must be unique.

If this is the case, I would look into handling this server side. You could create a new sql table which has username, region pairs. You could then pass in the username as a parameter to your SQL stored procedure and use that parameter to control which results are returned by the procedure. This is ultimately probably more desirable.