I activated my Nexus 4 with Straight Talk.
1. Visit Straight Talk website (https://www.straighttalk.com/secure/Activate)
Activate micro sim card and port your phone number
2.Phone will be ready to complete activation when your old phone stops working. It took about 10-15 minutes in the case.
3. Insert sim card
4. Make a test call & text message
5. Update APN setting
6. Runnung At&T HSPA+