Ok, so coming up with the highly variable selection of monsters in each encounter was sort of a challenge. I tried doing this the old fashioned way, but after a while I got stuck in an old habit I had used for AD&D- putting all of the same type of monster together in one encounter.
In 4E you get more out of variation (and they flirted with this type of feature a bit in 3E as well) and combining different types of monsters into one encounter. 4E encourages this a bit because it's easier to set up a multiple creature encounter.
For versatility's sake here's the (basic) formula for doing multiple combatant encounters in 3E: Take any CR creature and make it into two creatures, and you have a CR+2 encounter. Double it again, and it just adds another +2. So if one Orc warrior 2 was CR 1, two of the same orcs are CR 3. And 4 of those guys orcs is a CR 5. (and 8 orcs is CR 7, etc). This formula is most reliable between CR 4 and 12 or so.
The important part is- in 4E you don't think of individual monsters as an encounter by default, you have to think of a whole complex grouping.
The 4E DMG provides a neat bit of procedural advice for generating encounters in the form of the random dungeon generator near the back of the book. I highly recommend getting some index cards and playing around with that, and developing encounter decks based on a cast of likely inhabitants to a dungeon you design.. it does an interesting thing by recombining the individual monsters into cool groupings, and then you can sort of springboard/brainstorm ideas about why this group is working together, or perhaps they are both stuck in a clustered section of the map.
There's a free online version of this tool here:
Asmors 4E Random Encounter Generator.
Here are a couple of sample encounters that seem unlikely that we could build into clusters:
Level 1 Encounter:
3 Zombie Soldier (DDI) Level 2 Soldier
1 Stirge (DDI) Level 1 Lurker
3 Ice Warrior Shardling (DDI) Level 1 Minion
This one is pretty straightforward: the zombies are bodyguards to an evil spellcaster. The elemental warriors enter the encounter when a planar ward in the wizards lab is set off. The stirge just happens to live in the area (perhaps it is kept as a pet)- it can't feed on either the zombies or the ice warriors so it sees the adventurers as an opportunity meal.
Level 3 Encounter
1 Duergar Theurge (DDI) Level 5 Controller
1 Gnome Arcanist (DDI) Level 3 Controller (Leader)
1 Faerie Dragon Flitterwing, Adult (DDI)
6 Pack Zombie (DDI) Level 2 Minion
This one is much bigger and much more confusing: But I see it as this: The gnome and the fae dragon are adventuring companions despoiling the dungeon. The Duergar directs the pack zombies. No matter which group the PCs encounter first, the other group is sure to come along soon afterwards. I can see the adventureres fighting the duergar and his pack zombie mining crew, and then having the gnome and his fae dragon pal showing up secretly and playing pranks all through the encounter. '
Level 5 encounter
2 Dire Boar (DDI) Level 6 Brute
1 Kobold Wild Mage (DDI) Level 5 Controller
2 Rotwing Zombie Level 4 Skirmisher
This one is also pretty simple: The kobold wild mage controls the two dire boars as his pets- I can see them as a mobile encounter, perhaps patrolling on a path in the dungeon or adventure location- he might even be riding one of the boars. The rotwing zombies are either wandering or guarding a separate clustered area on the map. Alternately the kobold wild mage directs the rotwing zombies. The two boars are kept in a nearby pen..or they are free and wander into the area in search of food.
Level 7 encounter
2 Lost Wraith (DDI) Level 8 Lurker
2 Lasher Zombie (DDI) Level 7 Soldier
1 Shadar-Kai War Witch (DDI) Level 5 Controller
3 Duergar Miner (DDI) Level 5 Minion
Another simple one: The Duergar miners are an independent group, doing some deep mining, but they are allied with the War Witch who lives nearby. The Shadar-Kai War Witch has bound the zombies as her bodyguards.
The lost wraiths both haunt the collapsed tunnels. They are attracted by any sounds of battle.