This can be simulated in order to figure out the solution to having a ported enclosure that has the option of having the ports plugged for a sealed mode configuration similar. I must caveat this statement by saying that you could possibly be sacrificing the optimal ported size and thus performance, but, here is what you need to do.
Download winISD Pro, and pick out the driver that you intend to use, which will hopefully be an 18" or at the very least, a 15" and start modeling ported enclosures that are around 5cuft to 6.5cuft. Then just play with the different ported & sealed variations until you find both a ported size that works well down to you required cutoff frequency while at the same time modeling a sealed enclosure of the same size that works well without being too big, which is why I suggested 5cuft to 6.5cuft as that is what I believe you will end up with as a compromise. Think about it, if the ported enclosure is 11cuft or even 8cuft then that would be too big for four sealed version, but at the same time, it would be optimal for your ported enclosure, and vice versa, if you build a 4 cuft sealed box it would be optimal for sealed but terrible for ported. It may end up being more compromises than it's worth, just in my humble opinion.