If you purchas elder scrolls IV do you have to buy the other elder scrolls

Well, if your worried about continuety, then don't worry. They ALL have sperate plots.

If you want to buy the others for comparison, that's a different story. Try 'Morrowind', it is by FAR superior to Oblivion and then try the older titles - but you'll need a program called 'DOSBox' to run old games. Type it in Google, there's no sign in, no payment and is only a few kB in size.