Enclosure help 4 15s? Thanks

[MCG electrical services]

Hi guys, im new to this but wont bore you with a story! Finally sorting my new system but im struggling with the box, i have 4 jbl gto1514s not exactly a great sub but its what i could afford , they will be running off 2kw several batts and 200 amp alt all id like to get some advice on is... Internal area of enclosure needed and a good tuned port dimensions ? Is it as simple as 1 15 needed 3.5cu.ft so 4 need 14cu.ft? If not any help would be appreciated ... The port on the other hand really confuses me as on previous installs i had a 4/12 slot port 17" deep tuned to 33hz but in some calculators i have used its saying a 14 cu.ft box tuned to 33hz will need a port 4/8 and only 2" deep? So to sum it up any help on building an enclosure for my 4 15s will be appreciated , its going in a vauxhall vivaro van , walled so i have over 40"s height and width and all the depth in the world , thanks again