SOCOM 4 beta starting April 5. But you'll have opportunities to get in earlier:
- March 15 - Private Beta. "We'll be contacting eligible players soon."
- March 22 - PlayStation Plus early access
- March 29 - Access included with Killzone 3
- April 5 - Free public download on the PlayStation Store
Unfortunately, the beta won't give you a taste of the five-player co-op mode. You'll have to wait until April 19 to try that out.