UPDATE 1-Greece's Piraeus Bank sinks to Q4 loss, bad debt charges rise
* Piraeus posts 18 mln euro loss in Q4 * Full-year 2016 loss shrinks to 4 mln from 1.85 bln in 2015 * Non-performing loans drop to 37.5 pct of book (Adds deputy CEO comment, details) By George GeorgiopoulosATHENS, March 31 (Reuters) - Piraeus Bank