what is a good sporty coupe that is reliable?

A "good" " sporty coupe" "reliable" for around $4000-$6000 does not ever go together. You may find one if you look I'm sure you can find one but don't say good, reliable in the same sentence as your not going to find one like that for your price range.

I'd go with a Honda Civic (I like the 1999-2000 just because I think it looks cooler). It's relatively cheap (I've found plenty in your price range), it's good on gas (it has a small 1.6L four cylinder engine), and it's very reliable as long as you keep up with the maintenance (my grandma knows people who have driven Hondas for 300,000+ miles). It might not look very sporty-looking, but I'm sure you could find a cheap body kit that'll make it look a lot cooler. If you know how to drive stick shift then I'd recommend getting a Civic (or any car for that matter) with a manual transmission because cars with manual transmissions generally get better gas mileage and are more fun to drive.