Automatic 6 month extension by filing Form 4868 - Application for automatic extension to file:
A taxpayer can get an automatic six months extension to file his/her return(s). But the return must be postmarked on or before the due date of the return. Note the extension is only for filing the return and not for any taxes due. If any taxes are due IRS might charge interest and penalties.
When you file a federal extension form, it does not automatically extend the time that you have to file your state income tax return. Check with your state. However, CA allows automatic extension without requesting one in writing. If you have a refund coming and you are positive about it, you are ok. However, if you are unsure fill FTB 3519 and use it to estimate any taxes you might owe.