I came across a document from a Taiwan government site which suggests that BJ4140 was a call sign registered to the Taiwanese fishing vessel SHENG JI CYUN No.3, Official Number CT4-2240. Other images of SHENG JI CYUN No.3 show a very similar vessel, but with a different bow structure (Not outside of possible modification), but list the call sign as BJ4240, so it may just have been a typo on that one document.
At least it seems to indicate that "BJ####" is a format used for call signs on Taiwan-flag fishing vessels.