Okay, I'll bite: why wouldn't you just create a party using the 4e PHB?
I don't think this would be a good idea because of dailies, particularly at higher levels. Unless the DM is playing the NPCs as though they have to complete the adventure, enemy NPCs can blow through all of their dailies with little regard, creating an encounter significantly harder than it's level would suggest (because the PCs have to consider whether they will need their dailies later).
Regarding the OP's question:
Creating a rival party composed fully of elites is certainly an option. If the NPC party is the same size as the PC party you'll end up with a Level +4 encounter (which is quite dangerous but still potentially winnable). Alternately, you could make a smaller and/or lower level party of elites. The elite option makes the rival party a genuine and serious threat, though there is risk of a TPK (unless, of course, the NPCs aren't interested in killing the PCs).
You could also compose the party partially or completely of normal monsters. For example, on MM pgs. 162-163 you'll find the Human Bandit (Rogue), Human Guard (Fighter), Human Berserker (Barbarian), and Human Mage (Wizard). A perfectly viable adventuring party (though light on healing). If you don't want some of them to be human, reskin to fit (shifty for a kobold, etc). If they don't fit the level range you're looking for, level them up or down as need be, or if the gap is too large for that use them as inspiration for creating/reskinning much higher level "monsters". With this option the enemy party is likely to be a less deadly nuisance unless they are much higher level then the PCs or have some special advantage (they plan to hold their attack on the party when the party looks sufficiently softened up).
You could, of course, use both options. When they first encounter the NPCs the NPCs are elite and may very well kick the PCs around the schoolyard (though the PCs certainly have a fighting chance so long as they play smart). Eventually, though the NPCs keep pace with the PCs with regard to level, they lose their elite status (becoming "normal"). At this point, it is the PCs who will have the upper hand (and no doubt be pleased with how far they have come). If you need a way to rationalize the powers the NPCs lose after conversion, simply say that the party has learned to counter those tricks; they try them but fail miserably.
Edit: Or as Pseudopsyche mentioned, use the rules on DMG pgs 186-188.
YMMV
