Then the PCs won't be able to hit it. It would be frustrating, not fun.
That is not necessarily the case:
Use brutes of MM3 design!
1. Level +4 brute has Level +0 soldier defenses
2. Very high damage
3. A very good to hit chance
When you look at powers, defense boosting is easier accessible than attack boosting.
So in the end you will have about 50% hit chances on both sides.
My last encounter i dropped on my players was frustrating for me.
I used 3 brutes of level 4 against 4 PCs of level 2, with no preparation on both sides. Their alpha strike knocked out the bard, but they were smashed easily with the use of some dailies.
Note, that it was an even level encounter, but each individual was higher level and the rogue missed them only with a natural 1. So i guess using a brute of level +4 would have been perfectly ok.
(Note that brutes are often those monsters which you could imagine walking around on its own or small groups, like ogres, or a single ogre between some orcs.)
It was the MM1 monsters where you could not make a single challenging encounter because defenses were higher, because you had to use monsters of higher level a threatening challenge.
As i said earlier: the 3.5 mentality of an equal level encounter draining only 20% of the resources combined with high hp, low damage monsters was the cause of the gind.
With Level -1 combats already quite dangerous you can easily make combats that drain a lot of dailies without resulting in a grind.
(at least in theorie... I would at least give MM3 or MV a shot)
Edit: two elite brutes of level +4 should make a good challenge. Chose two that make more area attacks instead of double attacks or don´t focus on a single PC