can i change my passcode in i cloud?..beccause my iphone 4 has been stolen.

No.

If you turned on the find my iPhone feature from the iPhone itself, then got to iCloud.com and see if it can be located, or wipe the iPhone from there.

Otherwise you can do nothing but change your passwords, report to police and alert your carrier.

Again, no, you cannot change your passcode from iCloud.

If you have reported to police and your carrier and have changed passwords, then there is nothing more you can do.