How I can sign out Gmail in Iphone4

Hi Chaugiang!

You can follow the instructions in the article you have linked in your post to delete your Gmail account from your phone:

iOS: Troubleshooting Mail

http://support.apple.com/kb/TS3899

Delete the affected email account from your device.

• Tap Settings > Mail, Contacts, Calendars.
• Choose the affected email account, then tap Delete Account.
• Add your account again.

Or you can simply make the email account inactive by going to your Mail settings:

Mail settings - iPhone

http://help.apple.com/iphone/7/#/iph5ca4d6a69

Go to Settings > Mail, Contacts, Calendars, where you can:

Temporarily turn off an account

Thanks for coming to the Apple Support Communities!

Regards,

Braden