It's not realy a complex quety:
Leave your fields as is ; ID, Num1, Num2, Num3, Num4 take out Sum. (By the way: do not call your field "Sum', it's name for aggregate function)

So next time when you are need to display data from your table just use sql statement:
Code:
SELECT *, ([Num1]+[Num2]+[Num3]+[Num4]) AS TotalSum FROM
TableName

txtTotal.Text=Recordset.Fields("TotalSum")