^
Group 1 = You can only enter as a solo queue.

Group 2 = You can only enter as a party of 2 or 4.

The 2 groups will never be matched together. Think of it as different instances. Obviously this is how it should work or else there wouldn't be two options for Wolf's Den at level 50. Did you not notice that you can only choose the last option when you're in a party of 4?

For your scenerio, I am guessing that there is a fail-safe that will allow your 4 man premade to be put into the first grouping because there weren't any other 4 man premades at the time. You just proved my point anyways. The Premade of 4 CREAMED the pug team.