Let's say all classes are created equal, at more or less the same power level.
If a party of four PCs (your standard fighter/cleric/rogue/wizard) fights their exact duplicates, both at full power, would that be a medium fight? or hard? or deadly?
any help is appreciated.
I'd say Deadly just from the flavor text: "A deadly encounter could be lethal for one or more player characters. Survival often requires good tactics and quick thinking, and the party risks defeat."
Given exactly equal opposition, both sides have an equal chance of winning. So flip a coin: heads you live, tails you die... 
- Eck
Absolutely deadly. Think about it, if they're fighting to the death then one side is certain to be totally eliminated. If they truly are mirror equals then which side survives is just a matter of a coin-flip or (more realistically) the difference in your ability to manage their class features. Situational factors such as terrain, resources, and iniative will likely play a deciding role as well. As such it's extremely likely that the player characters will suffer some casualities.
Delazar78 wrote:
Yup, pretty much what the others said: Deadly.
Don't throw that at players unless they have found a very clear advantage. Otherwise, a fight like that is pretty much guaranteed to kill at least one or two players, and have a 50/50 chance of wiping the whole party.