NAS supporting plex and that can handle transcoding

My HDD in my computer has started sounding weird and I think it's time for me to get myself a NAS. My plan is to have the Plex Media Server running on it so I can stream movies and series to my phone and Chromecast whenever I want so I can keep my computer off.
Most movies I got are in the MKV fileformat and I wonder if this has to be converted or not, If not a weaker NAS might do but if it has to convert I might need a much more powerfull NAS.

Does anyone know what I need? And do you recommend any NAS with at least 4 slots?

http://www.wdc.com/en/products/products.aspx?id=1170

EX4 from WD, one of the best consumer NASes I've ever setup/used. It supports Plex, and has a decent bit of power behind it.