is the ネックレス blair wears in 4x02 the same as the one Chuck gave her in 3x18?

I think the ネックレス is different...

Nope it's not the same, but they do look similar from that screenshot あなた got from 4x02. The ネックレス in 3x18 was from Cartier, but the one from 4x02 was from Harry Winston.