Call Neito Research (1-888-412-5552) or email them at NietoResearchInc@aol.com. Tell them you would like to participate in a diaper study. If you get voice mail, leave a message, and they will call you back. Leave your name, phone number and your child's current weight/diaper size. They are currently looking for babies wearing size 4/large diapers. Participants that send an email rather than call will have first priority.
I have done this twice when Nicole was wearing size 2 and then size 4 diapers: each time they sent me a package of diapers (mega size that would last us at least a week!) along with evaluation form that I had to complete. Pretty easy and you can't beat free diapers and you also get a chance to give your feedback to the manufacturer. :)