If you're immediately post-op, swelling can make some depth seem to disappear temporarily.
DO NOT push too hard; you risk tearing the vaginal wall and even causing a fistula, which is very very bad. Gently rotate the dilator a little and press lightly, and relax as much as you can, but if you can't get past that point, you'll have to try again another time. It's probably the PC muscle, but in your case with only 4" of depth to start with... there's a possibility that you're as deep as you can get for now, and again, pressing too hard against the end is dangerous. If you're still having problems, contact the surgeon and ask.
I wish I could explain how I know when I've hit bottom, but the truth is, it took doing it a few times and just recognizing the sensation thereafter.