Do you create a customer before invoicing them? We always do that first, rather than just entering info into the fields of the create invoice box. That would solve the problem of what's in the SHIP TO field, as it will be whatever is in the customer's info.
Have you unchecked the box SAME AS BILLING? If it is checked it will automatically fill in what is entered into the BILL TO field. Perhaps if you save your invoice with it checked or unchecked (whichever is the opposite of what you have now) it will default differently?
Is this default of 4 hours in the create customer box or the create invoice box?
You could also try re-setting it by removing the SHIP TO field from your invoice layout. Save your changes, then add it back into the layout.
What version are you using and what OS?