Although this is a matter of debate, I tend to hold off until the 5th set of leaves before I add ferts and I think, considering your lighting, you should wait because under only one light your girl is going to grow a bit slower than normal. Good ferts formula to remember is that the N should be roughly equal to the sum of your P and K. Always give nutes at a quarter of the strength to begin with! Remember, it's better to give ferts a little later than to add them too soon. Like Smokin' said, add a fan to help with your stem. Peace.