I know the JD MKII is the one used by cruisers/battlecruisers and battleships. My question is how much cargo space it takes up. (outdated wiki says 900 which makes it usless on anything but Battleships)
Also: Is the maximum jump distance still 4 systems or that has been changed too?
Yes, that was removed. There was a revamp that altered the way the jump drives work. They now use Jump Drive Batteries instead of fuel commodities and you can buy them at any station in the ammo equipment shop. One of the welcomed things @sindroms introduced during his green days.
The jump drive batteries don't take up any cargo space, meaning ANY jump drive is useful.