Kind of hard to tell If we don't know what Settings3 and 4 is. However, if you hover over Settings3/4.Virus_Signature_Database_beConnectionString, right click and in the context menu click 'Go to definition' you'll probably find something that looks like this:
Public ReadOnly Property Virus_Signature_Database_beConnectionString As String
Get
Return _someVar
End Get
End Property
As you can see this Property has no Setter, this means you can not change the value of _someVar outside your Class (unless there is some other Property or Method that alters it).
You should either change the Property so that it can be set (deleting the ReadOnly keyword and adding the Setter), or find another method that can change it.
Perhaps you should look for something like:
Settings3.Connection.ConnectionString
.
Good luck and I hope it helps!