If you don't already, get a test kit for ammonia, nitrite, and nitrate (the liquid test kits, not the test strips or the 'meter' type).
Then start by dosing with a few DROPS (I would say no more than 10 max for your 1st dose). Then wait about an hour to give the ammonia a really good chance to evenly disperse through out the tank. Then measure your ammonia levels with the test kit. Since none of the ammonia is going to initially be getting converted to nitrites, you'll be able to do simple math to determine how much dosing you need to get to 4ppm.
For example, I wouldn't be surprised if those 10 drops gave you an ammonia reading as low as high as 1 to 2 ppm. If you get a reading of 1ppm, then you know you need to add 30 more drops. If you get a reading of 2ppm, then you need 10 more drops.
I found it convenient to place the ammonia in an old eye dropper bottle of some sort (like the bottles used for contact solution).