Four real seats?

[Lordblood]

It's really difficult to have a mid engine and 4 usable seats all in one car, in fact I'm not sure if it's ever been done, the closest in the 911 but thats rear engined.

I think depending on your price range, the BMW M3 (new or used) cabrio or the Audi RS4 Cabrio is the way to go. They don't have nearly the sportiness of a Boxster, but if you must have 4 seats this is the way to go.

Now if you can manage without a convertible top, there are a few more choices, the RX8 or the completely crazy Mitsubishi Evo are really good handlers as far as sports cars go.