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Skirmisher is just a game term for monsters who basically hit and run.  Elites are tougher versions of standard monsters.

Refer to page 92 in the GW rules book.  That is the section for "builiding encounters."

The gist of it is you determine how hard you want the fight to be and use the chart to figure out your XP Budget.  Once you have the budget you "shop" for a monster or group of monsters that roughly equal your budget.

To touch on your example.  Let's assume you want the fight to be a Standard encounter, not too hard or too easy.  Looking at the chart on pg 92, a standard encounter for 4 level 8 characters would have an XP Budget of 1400.  An 8th level elite monster has 700 XP or half the budget.  It would probably be a cakewalk for your group.

4 level 6 characters have an XP Budget of 1000 for a standard encounter.  Now we're only 300 short.  You would probably want to add a couple more monsters to get up to 1000.

In general you want to avoid throwing monsters that are more than 2 or 3 levels above the PCs.  Otherwise they'll have a hard time hitting it's defenses.  It might fit the XP Budget but your players will probably get frustrated.  There are other factors to consider like party makeup and monster types.  You'll have to play around with it to figure out what works for you and your group.