How many days pass during the first part of Konosuba episode 4?
Actually, if you look closely at that episode, Kazuma's explanation of the different setting was of weathers, not of seasons. It was meant to show that they trained regardless of what weather it was during the day. So, basically only days passed, not seasons.

To further reinforce this, Volume 1 Chapter 3 of the light novel directly states how much time passed. To quote, 


  Thus we spent a week in pleasant explosions. Then, one morning…