login - Is it better to force sign in before or during restricted forms?

I am building a form that will require a person to be signed in to successfully post. Filling out this for is ESSENTIAL to my web application, so I really want to make it as painless as possible - and, to make it harder, many people will try and fill out this form before they realize they need to have an account and be signed in.

Is it better to:

1. Prompt them to sign in / register BEFORE they even get to the form with a lightbox popup (ala Digg).


2. Prompt them to sign in / register BEFORE they get to the form with a separate page altogether?


3. Allow them to get to the form and present them with a lightbox popup?



4. Allow them to get to the form, and add the sign up / register fields to the form itself?

Answer

#4 is the best choice. You do not want to take the user out of the flow they have going in their mind.

They are focus on completing a task on your site and taking them out of that flow discombobulates them. The lose their train of thought and that makes users abandon or just causes displeasure in the experience.