Here are a couple of links that detail a recent change that covers Market Makers ability to trade out to 6 decimal places and how they are unable to quote it beyond 4.

Here is an overview of the changes that went into effect on 11/17:
http://www.finra.org/Industry/Compliance/Mark...RF/P470179

Here is the rule re: OTC quoting and trading increments:

http://finra.complinet.com/en/display/display...nt_id=9338