Pretty certain that the Type 054B frigate is from a decided PLAN 4000 ton displacement requirement, same as the 054A. That decision for the 054B may have been made early in 2016, picking from various design proposals, the losing ones eventually became what CSSC displayed in the defense show at Abu Dhabi as export frigates, which are pegged at 4000 tons. As usual, things and proposals that are not accepted by the PLA end up being offered for export.
Recent display of export submarine designs last year may also be an indication of a Yuan class successor already at work, and the designs hint to that, which maybe a single hull this time.