Well, Map 4 is now free.

My alliance is still growing and we can clear Map 3 (but not 100% exploration yet).
Is there any strategy / tips I can learn to decide if when to start doing Map 4 (and alliance not get discouraged; still can't 100% Map 3)?
What champs are generally useful?
What nodes are difficult (think I saw degeneration

)
Champ rank and level?
Thanks to all.