In case you haven't seen this link from other threads:
https://morewinemaking.com/web_files/intranet.morebeer.com/files/wyeastpair.pdf MoreWine doesn't look to have a recommendation for RC212 with Merlot, but I've used RC212 on my Stag's Leap Merlot, and it turned out well. Note that in this article, "BM45" can be substituted for "BM4X4". From my understanding, BM4X4 is an updated strain partly based off of BM45.
The yeast imparts a lot of flavor, so I think it comes down to personal preference. I've used both RC212 and BM4X4, and I think BM4X4 produces a taste that is closer to the commercial wines I like, but your mileage may vary. Either is a good choice for Merlot, in my opinion.