Hi - I went the year before last. It is pretty in the early evening as all the lights come on. There isn't all that much there, a nice little Christmas market (pretty much same stuff as you get almost anywhere at Christmas though!; very busy ice-rink and a few rides. It is quite expensive - can't remember exact costs but remember thinking that I wouldn't pay that much to go on the big wheel etc. Would probably be a nice start or finish to going to the oxford/regent street area to see the christmas lights though!
I really didn't think very much of it at all. Everything was very expensive, and it was crowded and hard to get somewhere to sit to eat or anything. I was expecting more of the market which was very limited. It seemed like a hyped up, overpriced fairground more than anything. I'm not sure it would be great for a 4 year old at all.
took dd last year (she was 3.5 at the time) she was too small to really go on anything and it was freezing - there was nowhere to go to get out of the cold. i wont go again this year. there is a xmas market/santa thing on at the south bank this year, that looks better and there are places there to get out of the cold/get proper food. www.viewlondon.co.uk/whatson/santas-secret-village-london-article-7408.html
I went last year with my two (grown-up) DDs and we did enjoy it, I must say - the Big Wheel was very nice (not as good as the Eye, obviously) and it somehow all felt very Christmassy on a cold and frost December afternoon as dusk was closing in.
However, I don't think it had a lot to offer a 4-year old. Drop in if you were passing, perhaps - he would probably enjoy watching the skaters - for 5 minutes! - and beg you for something from the Market (quite pretty, done in little Chalets) but not worth making a special effort to go.