hie guys,just wanna ask , I am using Iphone 3gs now.So i can upgrade to OS4 right?

OK so i have downloaded the latest Itunes ,and when i connect my phone to pc ,everything is ok.

When i click on my iphone icon on itunes,it shows " Check for updates",so i click that but it comes out "THE VERSION OF THE IPHONE SOFTWARE 3.1.3 IS THE LATEST VERSION"


So how to update to OS4 la?