If you want to experience SOCOM 4's 32-player multiplayer battles first, buying Killzone 3 is the easiest way to get access.
This from Zipper Interactive's Chris Roper:
"To get early access to the beta, look for the SOCOM 4 callout on the front of all eligible copies of Killzone 3 (including both the standalone and Helghast Editions). Inside the box you'll find a PSN code that you’ll be able to redeem for your early access as soon as the beta goes live. To find out when the SOCOM 4 multiplayer beta begins, be sure to check back at SOCOM.com once you pick up your copy of Killzone 3, hitting shelves on February 22nd."
Here's the box that you're looking for (notice the SOCOM dog tags?):
Buy Killzone 3