I don't know that I can help much as that's about how far I throw (your pull line is probably in a better position than mine too).
That said, it looks like you need more weight forward and/or pull a little later (you appear to be really neutral at release). You need to get up off that back foot earlier. It's still touching the ground after
the disc has left your hand and your plant foot is around 45 degrees.
Someone with a keener eye could tell you this for sure, but it looks like you're dipping in that left shoulder early. I got the Youtube video to stop at a frame that suggested this, but now I can't seem to find it again. I wouldn't worry a whole lot about this right now - it's a timing thing and not easy to change (I do this too and it's damned difficulty to concentrate on). There are probably bigger fish to fry at the moment.
I would guess you'll hit 400' by transferring the energy you're losing from your back leg - even if you transfer it inefficiently at first. Of course, it'll confuse your current timing too.
Good luck, keep us posted.