You’re really not giving enough information to be helpful. If you’re clicking on a link we need to know what the url is that the link was supposed to go to and we need to know what the url is that the link actually takes us to. Saying you’re getting a 404 doesn’t really add much!
It’s in your settings. Log in to your admin section, click on “Options” and then “Permalinks”. Then take a look at what the settings are in there and report back. It’s possible that there’s a screwy setting there.