Q: I Lost my iphone 4s yesterday.

If it is in someone elses hands and did not have a lock code on it originally, they may (if they know iPhones) have simply gone in to your account settings and disabled the find-my-iPhone setting.  If they did that, they have full access to your device and anything you try to do remotely from iCloud will fail (did you at least have a passcode set on restrictions, and dis-allow any changes to accounts?).  If there was no security enabled on it at all, then they have full access and use of your device, which means they can simply disable any remote iCloud features.

You should immediately change any and all passwords to any sites you used from you iPhone.  Also call your service provider and make sure the account is cancelled.

Even if the lock code was set before the theif did anything, all they need now do is plug it into iTunes and restore as new.  That will at least wipe all your information off the device, but they can then go ahead and activate and use it with another account.  Nothing you can do about that.

File a police report in case you need it for any insurance claims you may be able to make.

If you lock it by remote, the phone will be locked as soon as it gets online or is detected using the network.

Have a look at this article about the remote lock feature: iCloud: Troubleshooting Find My iPhone, iPad, iPod touch, or Mac

Also contact your provider and report your phone stolen, some can put it on a blacklist.

Reporting a lost or stolen Apple product