Ok I'll go from 3rd to 4th to new Tau Codex.
3rd - ALL attacks go to the drone at that init. If you had 10 guys with init 4 and attacked a CS w/ Drone all 20 attacks are taken on the drone.
4th - Regular rules for close combat. Majority is where the wounds are taken first.
New Codex - Regular rules for close combat but drones have the same toughness and save as the unit they are with. Tau player gets to pick where the wounds are taken.
Shooting
3rd - Drones take the first hits.
4th - Majority toughness and majority armour save take the first hits. Now this was big because if you had 3 CS's and 1 Shield drone and a Dev squad fired 3 Lascannons at you and wounded you'd lose 3 CS's.
New Codex - All drones share the same save and toughness as the unit they are with. This way the above Dev squad wouldn't just kill off the CS"s and the Shield drone just sit their and not do it's job.
* New Codex was made so that the Tau would use the Majority Toughness and Armour Save rules.