The basic tool for gauging encounter difficulty is xp available from the encounter. For your five fourth level characters, that gives you the following values:
That's per character, so if you want your encounter to gauge out at medium difficulty, use monsters that will add up to 1250 xp. Depending on the number of monsters, you increase their effective xp value without actually awarding more xp. For example, if you use two monsters, multiply their xp by 1.5 to determine their effective difficulty. The idea is that more targets is more difficult for the party to handle than just one thing they all get to beat up on.
For more detail, refer to pp 81-85 of DMG.
(Sorry this was so late, I hope it helps going forward)