I have a basic set up in my basment closest and only have 1 plant going at the moment. its about 4 weeks old and is about 4 inches tall when should i start fertilizing and any suggestions would help thanks

And when should i switch to 12/12

You can start fertilizing now if you feel it's necessary. Just remember to introduce the nutes gradually. Most people suggest 1/4 strength.

well i havent seen any real noticable groth in awhile and i was thinking that fertilizing might be the problem

Yeah, go ahead and fertilize then. Anything older than 3 weeks will be happy for any nutes you give it. Just don't shock it with full strength.

As for switching to 12/12, it's a matter of choice. You can let your plant grow to be a couple feet tall if you want more bud, or you can switch to 12/12 when it's 8-10 inches for a quicker harvest. I don't think anyone would recommend flowering at anything less than 8 inches or less than 3 sets of nodes in height.

What kind of lights do you use and plan on using for flower?

a single 20 watt acarium light

Well I don't know that much about aquarium lighting, but I doubt 20 watts will be enough to carry your plant all the way through flower to yield an amount you would be happy with.

its flurecent and i figured that wasent enuff but i have a couple of 120 watt energy saver bulds thats i thought would be good but i have to set them up.

any suggestions for fertilizer though

I use 24-8-16, Miracle Gro for veg. Anything with a high N count is good. If you're absolutely not sure what you're looking at when you go to the store, just get some 20-20-20. It's a good all-purpose fertilizer.