Matching values between two different fields, each of which have multiple values
You shouldn't use delta for such thing as data sometimes get reordered in the tables (ex: for node reference, the nid gets reordered regarding the title of the node referenced).
So I'll recommend you to create 4 fields for images and 4 for the URL and eventually put them in groupfields.  

Also to make things prettier you can theme your form and put each group of field in a row using div or table.

Have a look at Theming Forms in Drupal 6 for information regaring form theming and the official documentation: The theme registry for special cases.

Another nice tutorial: http://www.akchauhan.com/theme-drupal-form/