Reduce the number of opponents. Preferably by reducing it by one normal monster of the party level per player less than 5. Alternatively, you can reduce an elite to a normal mob (half hp, reduce damage/crowd control). If you reduce an elite, do one for every missing player below 5, or if it's an elite, reduce hp and maybe the damage it does, about 20% pr missing player below 5.
The building blocks of 4e encounters are:
Minion 0.25
Normal mob 1
Elite 2
Solo 5
Add up until you get an amount that equals the amount of players you have. Make the encounter harder/easier by increasing the level of the mobs by 1-2. Rarely use mobs higher/lower in level than that, compared to the party.