I'm not an expert on bps but from what I understand with 4ths you want to seal in whatever qtc alignment you normally would as if it were just sealed and the make the ported side either 2 or 3 times the volume of the sealed side tuned to the resonance of the sealed side. The lower ratio would yield a flatter response with less peak spl and the higher ratio would be more peaky, but overall louder. I would think 4ish on the sealed side and 8-12 on the ported side. The problem with this is that it may peak too high, as a bl would likey be much more suited in a quasi, or series 6th, than a 4th.
Again I'm no expert on bps so please don't start building off these statements unless it's experimental only.
Bought from - Hazardous0388 (4 db Drive 2000.1s), Psychoacoustics (4 Powerbass XA 3000'ds), and blackedoutavy (PPI A200)
Sold to - D-Rew (Knu 1/0), d77543020 (Knu 1/0), F1McLarenAGR (Audioque 4x90), and SacTownBully84 (Knu 1/0)
Traded with - SacTownBully84 (my xa3kd for his xa1.5kd + cash)