The Jaguar Mark IV, pronounced mark four, was a saloon (sedan) car built by Jaguar from 1945 to 1949.
It was a relaunch of a pre World War II model made by SS Cars Ltd from 1936.
Before World War 2 the name Jaguar was the model name given to the complete range of cars built by SS Cars Ltd. The saloons were titled SS Jaguar 1½ litre, 2½ litre or 3½ litre. The two seater sports car was titled the SS Jaguar 100 2½ litre or 3½ litre.
After the war the company name was changed to Jaguar Cars Ltd. While the post war saloons were officially the Jaguar 1½ litre, 2½ litre and the 3½ Litre, the term 'Mark IV' was applied by the trade simply to seperate them from the officially named Mark V.
http://www.saloondata.com/ is an online site that keeps track of all the Classic Jaguar Saloon's still around.
You can also get a ton of info on the early Jags here:
http://www.jag-lovers.org/firstcats/mkiv.php3