Per MB Trading, the 0.004/share uses the following order routing:
"1. MBTR is a new order route which offers the same award-winning functionality as MBTX. MBTR utilizes specific liquidity pools without the traditional costs of ECN pricing models to seek "best execution." This new route offers the flexibility and functionality that has made MB Trading a leader in trading technology."
I wonder if there is a tradeoff of better commissions on orders of less than 4000 shares at the expense of poorer order fills from a more limited liquidity pool?
Also, from a MBT footnote, the SEC and specialist fees add to all commissions plans.