I wouldn’t hazard a guess as to where I am 400m-wise right now. I felt pretty strong on the first two reps yesterday, then form started breaking down. I’ve lost a lot of track time over the past couple of weeks due to the snow and my travel schedule, so I’ve focused more on drills, plyos, lifting, and low impact tempo (i.e. bike and pool work) in order to minimize my losses, and build some extra strength.
I look at workouts like that one as more of a conditioning session than anything else. I have a much tougher workout scheduled for tomorrow (250m, 200, 200, @ 24 sec pace/8 min recovery) which should give me a better idea.
In terms of your 400m time, the usual estimate is (200m PR x 2) + 4 seconds. So you should be looking at a time of 48.4ish. It doesn’t always work out that way (hence the disparity between my 200m an 400m PRs), but it at least helps set a realistic goal time for you. Do you have any 400m races coming up this season?