I'm guessing Luxury/Performance, so I'd recommend the following: For a little bit more than 50K you can get a bare bones M3 Sedan or a C63 AMG.
But if you want to save money, you can look at a 335i, and then after you buy it, spend about 1K on ECU mods. Straight-line performance you'll be a little bit quicker than an M3.
But you really need to tell us what you want you like, since pretty much the only thing you said it that it needs to have 4 seats and be under 50K.