They are only 2 weeks into flower and they are 4 feet tall... Depending on the strain they will grow by at least another third. If they are a strong growing stavia they could double and even triple in size! If the leaves are narrow and skinny then you have a stavia dominant strain. If they are broad and leafy then you have an indica dominant strain. Most indicas do not grow nearly as big as their stavia cousins. If they are stavias you may end up having to cut them down. If they are indicas you will be OK if you start tying down the main stalks. I recommend you start learning about and implementing Low Stress Training (LST). If you are going to LST you need to do it now. Two more weeks and it will be too late. You will also want to do some thinning / pruning if you haven't already. Read this tutorial. It will help you. Post up some pictures.