Because two of the discs in the fourth season DVD set have missing laugh tracks, Paramount is making replacement discs available that include the missing laugh tracks. To get your replacements, you must do the following:

Remove the Proof of Purchase tab from the Fourth Season box.
***New info: It's the top flap inside the spine of the box that's revealed when you open up the spine. If you've already sent in the white bar-code on the bottom of the box to get your replacement discs, that'll work just fine, too, but the Proof of Purchase on the inside of the box will probably be even gentler to the appearance of the DVD box.
To clarify, the 'back' (side opposite of where the DVD cases slide into the box) opens like the top of a box. Open it up and you'll see the proof of purchase right there in that top tab.***

Mail the original tab with a letter requesting the replacement discs to:

Paramount Home Entertainment Customer Service
"The Andy Griffith Show" Fourth Season Replacement Discs
5555 Melrose Avenue
Hollywood, CA 90038

In addition to enclosing the original Proof of Purchase tab, include your name, mailing address, and phone number in the letter. Do NOT send the discs themselves. Allow 6 weeks for shipping.

edited 4/25/06 to add new info - this is all from Miss Crump over at her board:

http://www.tagsrwc.com/crumpbb/messages/9957.html