Hi,
You have 2 possibilites:
1. put table1 as record source on form. Then right click on border between main form and child form. On properties go to data tab and chose link child field or link master fields. New form will open. On this form deternime the link (i assume its No?). Click ok, close properties, save form and it should work.
Remember that you should have some other table on relationship with table 1. But the idea is the same
2. Create a search field, on main form or use your combo box afterupdate event.
Put a link to main form field in subform WHERE clause (you can do it with query builder too). And put this code on afterupdate event on combobox: Me.Child4.Requery.
Hope that helps you.