The rule of keeping birds happy is to get the largest cage you can possibly afford.
There are many different brands or types of ''6 foot tall 3 foot wide flight cages'' that will hold 4 cockatiels easily. They usually come with 3 perches and 4 food/water dishes. Sliding doors to place food or water in and a entry door available on the side of the cage if you need or want to hook up a breeder box to it. They have sliding trays to give you easy access to slide out the tray and clean the cage and has a grate that sits over the bottom tray to keep your tiels from eating droppings or old food that falls to the cage floor. They roll on wheels and have a sturdy shelf on the lower part of the stand which gives you plenty of space to store all their food toys and stuff!
Prices vary from $150 and up ... depending where you live. Shop online and order and assemble it yourself. They are easy to put together and last years.