To get the Airship back on disk 4 you have to jump through the gates halfway down the chain that leads to Ultimecias Castle. One of which will allow you to access a Chocobo Forest. Once you have your Chocobo you need to goto the tip of the southern desert nearest Esthar. You'll find the Ragnorak just sat there waiting for you.

However you can't access any Towns orCitys once your on disc 4. You can enter the Chocobo Forests but I think thats all. Can't remember been a few months since I played FFVIII.