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The 4th team wasn't an addition, but rather a relocation. The Atlanta Thrashers moved to Canada to become the 2nd iteration of the Winnipeg Jets. The original Winnipeg Jets had moved to Arizona to become the Phoenix/Arizona Coyotes.

How did they keep the conferences even. If you only add 3 teams isnt one getting more than the other?

There have been several occasions when NHL had lopsided conferences & divisions. Recent expansions have taken place 1/2 team(s) @ a time. Moves can also shake up conference/division alignments. NBA & MLB also suffered from this problem.