Lots of guns at that price.
I don't have any experience with the BPS, but I will give you some other recommendations/advice.
My personal favorite is the Winchester 1300/SpeedPump. They have a nice, smooth action and are great for follow-up shots. They haven;t made them in a few years, but if you look, you will find a few nice examples. I even come across a New-In-Box guns from time to time.
You will probably get a majority of recommendations for the Remington 870 as well. They are excelent guns as well, a little stiffer of an action, but the design has definitely lasted the test of time. I think I just read that they recently produced their Millionth 870. Just DO NOT GET THE EXPRESS MODEL. I personally like the old Wingmaster 870s but thats neither here or there.
Mossberg 500/590 whatever. I have had my share of problems with these guns. After dealing with a few of them, I will never again. I also don't care at all for the action either in operation or design/assembly/disassembly.
There are other good options as well, but these guns make up a good portion of my experience with scatter guns.
$400 should treat you very well in the used rack for a nice gun. Good Luck!