The first 3 instalments (all on DS) sound Sega Genesis/Mega Drive-like. I feel like Atlus copied sounds from Sega Genesis games and used them as music in the DS games of Etrian Odyssey. Battle theme 1 in the third game is the best!!! Now imagine Etrian Odyssey/Seikaiju no Meikyu OST GBA remix, like with a generic soundfont or Pokémon Ruby or Fire Red. The newer ones FM synthesized.
The first one sounds like a boss theme, the second one I already explained, the third one sounds like a battle on sea and the fourth one sounds like an alien attack/invasion.