Will adding more RAM fix 4K transcoding buffering in Plex?

No. 4K transcoding buffering is almost certainly a CPU or GPU problem, not a RAM problem. Transcoding is compute-heavy, and more RAM won’t make the encoder go faster. 16GB is plenty for a home server running Plex; a single 4K transcode only needs a few hundred MB. Instead, check your CPU’s PassMark score (Plex recommends 12000+ for 4K software encoding) or enable hardware transcoding.