I figured I can jump in on this. I noticed you are talking about Tlines as well. If you are looking for numbers, do not go with a tline. They can produce great numbers, by all means, but the response is geared mainly towards a broader response curve. Even a simple ported bass reflex design might do well for you, depending on certain parameters that need to be looked at. Im sure the 4th order will do great, even a 6th can be figured for a concentrated freqeuncy range about 3-5Hz band, which some prefer if they are unsure of their vehicles peak resonances.
But overall, if you want to do a 4th, then do it. As far as helping with the actual design, that is not something I can do in a post, lol. It will take a lot of work to get it right. You can get something great with general information, as we are talking about LF response in a small area, but fine figuring the design will do wonders for it. Just adding that in there for thought.
Oh, and Proper tlines do not inherit a tuning frequency, more so a low impedance dip depending on the phase of the enclosures mouth in relation to the driver position, but again I would not recommend a tline for that, so do not worry about that.