Why does iTunes want to charge me again for previously purchased music

If you mean that they aren't showing in the Purchased tab in the iTuns store app on your phone for redownloading, then have you checked to see if you've hidden the other 7 tracks from the cloud : iTunes Store: Hiding and unhiding purchases - Apple Support ? If they aren't hidden then you're sure that you bought the whole album, not just those 4 tracks ?

You are looking in the Purchased tab in the iTunes store app on your iPad, that is where you are trying to redownload the tracks from ?

If they don't show then, and if you've never used iTunes on a computer on which you could have hidden them, then it used to be possible to hide items on iOS devices as well - though I don't think that it was ever possible to unhide them on a device

On my iPad my purchased albums show the album and track prices in the store, they don't show Purchased against them.

If you go into Settings > iTunes & App Store and set Music (under the 'show all' heading) 'on', do the tracks then show in the Music app with a cloud icon against them ? If they do then tap on cloud icon and they should download

Do you have access to a computer with iTunes on so that you can check to see if they are hidden ? If not then you could try 'buying' one of the tracks from the store and see if you are charged

If the album has been removed and replaced with a different version then you won't be able to redownload it - they will be treated as two separate albums and therefore purchases.

You should be able to copy purchases from your iPod to your computer's iTunes library via File > Devices > Transfer Purchases

Have you checked to see if the tracks are hidden : iTunes Store: Hiding and unhiding purchases - Apple Support ?

If you do have to sync them then use drag-and-drop from your computer's iTunes to put them on the iPad, syncing may result in your existing content on the iPad being replaced with what's in the iTunes library.

In Edit > Preferences in iTunes, on the Store tab there is a tickbox for 'prevent iPods, iPhones, and iPads from syncing automatically' - tick that (preferably before connecting the iPod for the 'Transfer Purchases' above) :

And to copy music to a device, select/highlight the music in the My Music part of the library and then click-and-drag it over to the left hand side/edge of the iTunes screen - you should then get a sidebar appear listing the device(s) that you have attached to iTunes, and if you drop the music onto a device it should be copied onto it.