Do you have to do all 4 attractions, or can you choose which ones to do?

You are charged one price for all the attractions so you might as well do them all ,but you can pick which ones you want to do except for the hayride which brings you towards the other attractions

I do believe you can opt out from doing area 51 and the total darkness area

No you can skip any attraction you want with the exception of the attraction that is within the hayride.

Once you complete the first attraction you exit into the line for the next attraction until you have completed all 4. You may be able to opt out of one if you want but i don't think you can pick and choose.

You can skip the blackout room if you want but you still have to pay for all 4 attractions.