And can you come down during 1 or 2 weeks over the summer and be out every night, I mean so the bars ain't closed or something like that, is that possible?
Stay away from Poznan and Wroclaw during the summer if you want this - they'll both be dead in terms of nightlife. During university term time, both will be busy every night if you want to go partying - but not during summer. Poznan is almost like a morgue during the summer months, and Wroclaw isn't much better (unless you want to party with elderly Germans).
Can't talk for Warsaw or Krakow, but I'd assume both of those would have plenty happening every night regardless of time of the year.