There's no problem in switching from what you're currently doing to dosing ammonia. You already have a little start going and the simple ammonia will just keep things going.
The easiest thing we can tell you is that if you are in the UK then Boots ammonia is usually easiest to find, or Homebase. In the USA its Ace Hardware. If you don't have these places and have to resort to more of a search then you are looking for simple aqueous ammonia (it will usually just say "ammonia") that has no dyes, fragrances, surfactants or soaps in it. If you can see it, it should be clear and if you shake it, it should only create bubbles for 2 or 3 seconds, just like water. It should not create foam that stays.
Once you get the right ammonia it will often tell the percentage on the bottle, such as 9.5% or similar. This means you can plug the info in to the web calculator found in our "Calculator" right on the page you are reading. Be aware that ppm and mg/l are the same units for our purposes. Once the calculator tells you the number of milliliters of your concentration of ammonia for your particular volume tank then you'll want to come in a little lower than that, wait 20 min for mixing in the tank and then test to see if the result looks close to 4ppm. If it looks a little light you can plan to bring it up to what the calculator said next time. The important thing is not to dose the ammonia up at 8ppm as this will select for the wrong species of bacteria.
Does all that make sense?
~~waterdrop~~