I'm 4 weeks out of ACL reconstruction. Should I be able to reach full extension by now?

YES!! Gaining full extension should be the first thing that you work on achieving. Depending on the type of graft you have it may be more difficult to get full extension but it should be your first goal. Range of motion followed by strengthening and sport specific activities is the general course of your recovery.
Acl. You should be able to reach extension by 4 weeks. If you cannot try prone leg hangs with a weight on your ankle or an extension board.
Yes! At this point, it is extremely important to get full extension. If you cannot get it with aggressive exercises or with aggressive physical therapy, there may end up being a need for surgery to remove scar from the front of the knee that cannot be removed any other way. Prone hangs, weights, and assisted manipulation may be necessary at this point.