Actually for best results you would be much better flowering 4 when they are about 8,9 inches tall, that would be much beter use of your space and light.
i wouldn`t advise you grow them much taller as the light penertration with a 400 isn`t that great.
Plants have there bigest growth spert in the first 2-3 weeks of flowering i started when my grapefruit were 10inches and there over 4 foot now which is to big for reasons already stated.
You would be much better of flowering more than 1 believe me.