BSPlayer presently shows max 3 lines.
Living in a "real subtitle-country" I've hardly ever seen subs of 3 lines (e.g. on TV, cinema and/or commercial DVDs) and surely I've never ever seen subs of
over 3 lines. In my opinion decent subs are max 2 lines, otherwise they lose their function.
http://forum.bsplayer.org/viewtopic.php?p=11844#11844
P.S.:
This is just my opinion; perhaps
bst, who is the maker of BSPlayer, will nevertheless have an open ear to your wish and increase the present maximum of 3 lines shown. Well have to wait and see!