how to validate the last four digits ?

KewiDK 
Apart from you last line of code, there is also some typos on some line,

1. Corrected the variable name: You declared a variable from but used frm in your code. I corrected the declaration to Dim frm As Worksheet
2. Removed the CStr function: The CStr function is not necessary when you’re getting the text from a range. So, I removed it and directly assigned the text to strValue.

Here’s the comparison for clarity:

Your code:

Dim from As Worksheet
Set frm = ThisWorkbook.Sheets("Enter data")

Dim strValue As String
strValue = CStr(frm.Range("I6").Text)
...
Than If

here's what I did:

Dim frm As Worksheet
Set frm = ThisWorkbook.Sheets("Enter data")

Dim strValue As String
strValue = frm.Range("I6").Text
...
End If