Yes, you can pick more than 4. But I don't recommend it, because then the post tends to get lengthy and link filled. I do recommend including at least 2 links for each site you pick. One to the main URL, and the other to a favorite post of yours from them.
In the title of your post, even if it's just at the end, include the hashtag #payitforward.
We encourage you to do this on a regular basis. Not just for bloggers, but anyone that needs help getting the word out about something, especially if it's something life altering.
When you're done, link it up below, and go let the people know that you wrote about them!