Typically, plant size on tags is given as the expected size at 10 years. How soon do you want your plants to start growing together? If it's at 10 years, then go for the 4 feet. If you want it sooner, then you'll want to plant them closer, but in the long haul they will have less definition as individual plants.

Hi i know its not related but im having difficulty working this forum start point out,
So im looking for help on my new azaleas, they are still flowering from the last season its mid summer now n 30-40% of each plant has the branches turning to elongated woody stems with new growth on top, i want to prune them but I was told its to late, but if i dont de wood them i will loose the hedge shape im growing them for.
Can i prune out the long woody tufts n exspect new growth to shoot out?

Azaleas can be thought of as a groundcover, albeit high, in the design world. In that case, closer is better to avoid that individual look you see almost everywhere. Naturally, they are a groundcover of sorts.
Balelf, the answer is yes. You should start a new thread for further information and also list your Zone and general location for more appropriate responses, otherwise all answers are dealing with generalities.
Mike