BOTH!!! Use both of the lights. Both spectrums are very bennificial for flowering. I have several 4ft 45watt flouros mounted on my walls just to add spectrum. By adding extra light, not only will you increase yield but density will increase as well. I say get as much light as you can in the flower room, as long as you dont deplete the CO2. But as long as you have a decent size grow room, CO2 shouldnt be an issue, and definantly if you only have 4 plants. Oh! and 1 other thing, Ventelation and air circulation are going to need to be taken into account when adding another HID light.

Hope this helps!

And if you are 4 weeks into flower, this should be the end of their height growth. Now they should just bush out.