tiferes mordechai- new place between flatbush & boro park 600 mcdonald avenue, a bit on the smaller side but could probably fit 400 guests comfortably, dance floor not so big, but again if you’re having 400 it probably fits your needs, beautiful decor- they really focused alot on the interior, which you don’t find with most halls. i was only at one wedding there and service and food were fine, like i didn’t notice anything blatantly bad
ateres chynka- located in flatbush on elmwood avenue off ocean pkwy, much larger and their minimum is like 500 so might not even be an option for you. huuuge dance floor and dining hall, we fit 530 very comfortably and with big dance floor. i go there often for weddings and love the food, and we made a chasuna there ourselves and the service was great, the whole process very organized, they answered all our questions, and alot of ‘extras’ (nicer meats at smorg, choices at meal) were included at no extra cost, it was part of the pkg deal. we were very happy there, but again you have to meet their minimum. the only annoying thing which is a very small detail is that they have a custom-fit canopy on their chuppah and they dont want it to get ruined so they dont let any florist put ANY flowers or toulle or anything on the chuppah, so it looks very plain, but we just put nice flowers in the front and it looked fine- not a reason to not use the hall, im just mentioning it.
you may want to look into tiferes rivka, another new place that just opened.
mazel tov and good luck!