Automaticly selecting check boxes

Hi, here is the situation:

ive been trying to get this to work but no luck...

Text Box: Email

Check Box 1: newsletter
Check Box 2: promotions
Check Box 3: HolidayCard

Check Box 4: BadEmail

Heres what i need:

if any of the checkboxes (1, 2 or 3) is checked, and there is no text in the text box, then checkbox 4 will get checked...

however, i would still like a manual option with checkbox 4

thanks in advance for all your help...

shy