When to flower?

hello i have some pretty girls i need to flower.. or at least i think they are 40 and 50 centimeter's tall. is now a good time?

I believe that it needs to have at least 4 or 5 nodes.

(nodes)? what is that.. sorry im a noob

Well thats 15.5" for us in the states. If it has been at least 4 weeks go ahead switch to flower. I hope they are all girls. Nodes are where a set branches come out from the main stem. As your plant grows a new set come out thats a node

Do they have to be 4 weeks for any reason in particular? I have had some plants for about 2 weeks, but they are under 1000w and look amazing. They are about 15" tall and have more than enough nodes. They are clones that i got from a cannibus club.