Typically a group of 4 level 1 players should be able to handle 3-5 level 1 monsters. But there are many factors that make that slide around. Player skill, Group makeup, environmental conditions and props, etc. All can affect success/failure and balance. If you have monsters that are susceptible to Fire, and a player group has Fire to dish out, they can handle alot more enemy power. When it comes to level difference, generally what I find is that adding a level to a monster requires adding an opponent to overcome it. Again, its slippery. But its a rule of thumb.
One thing I had fun with last night while trying to balance an encounter, was I dropped 4 Archetype NPCs in against an encounter I setup, and I just watched the AI fight the AI in the GM window. I played with different damage types and levels and numbers, etc and actually had alot of fun seeing it play out, but I also learned alot from it. Try it out, but remember, players are smarter than that AI, so whatever outcome you observe with this technique, just know that the players will probably fair a little better than this.
