CAVEAT: I don't have Pathfinder and the CR/EL rules may have changed a bit since 3.5, so forgive me any erroneous assumptions I'm operating under.
I'm running a 4th level campaign that my group hasn't played in about 6 months, so it's been a while since I have DMed. I am trying to give a nice variety of challenges, some easy, some challenging, but my problem is what's a good challenge for a party at 4th level. I read that doubles of the same monster adds 2 to the CR. So under that logic, 4 cr 1 monsters should be a challenge for a 4th level party of 4. And that doesn't seem right to me. Any thoughts or suggestions on this?
Um, first of all, 4 CR 1 monsters is EL 5, not EL 4 (1 +2 for doubling to two creatures +2 for doubling again to four creatures). A group of three CR 1 monsters is therefore EL 4.
Second, remember that an "appropriate challenge" should deplete about 20% of the party's resources- spells, hps, etc. So don't mistake an "appropriate" challenge for a tough one.
Finally, CRs and ELs and all that are an art, not a science. Especially when you use npcs with pc levels- note that a 10th level sorcerer is CR 10, and there are prolly examples of CR 10 monsters with the spellcasting abilities of a 10th level sorcerer on top of their other abilities. So CR and EL break very easily when you try to use them as a real yardstick to measure difficulty, but work very well when you recognize that they give only approximately correct ratings. (Plus, compare the real difficulty of fighting a wraith when the party has a cleric vs. doesn't, or a golem vs. a party with no fighters or all fighter types- the same monster's "degree of challenge" can vary a lot, depending on the pcs.)
All that said, if you want a challenge for your party that uses CR 1 monsters, choose a good mixture of CR 1s- perhaps a group of barbarian 1s with Toughness for extra durability screening a 1st-level sorcerer with tons of
magic missiles or
orb spells, or maybe a healer. Generally speaking, however, the bigger the difference between your party's level and the CR of the individual monsters, the easier of a time your pcs will likely have.