I don't think your tank is cycled if you are getting nitrites. Now that fish are in you will be doing a fish in cycle. 4 ppm nitrites is lethal levels for fish. Start out with a 50% water change. Then do another 50% water change around 6 hours later. Test your water a couple hours afterwards and see where it's at. If it's still high, anything over .25 ppm, then continue with water changes until it's within acceptable levels. Remember, if you have 4 ppm and you change out 50% of your water you will still have 2 ppm.