I'm going with the 4000 if the price difference isn't prohibitive. I've used both and the 4000 has a few things that I like better about it when using it. I own a 3000 because it was a gift and it does work well. If I used it more often I would get rid of it and upgrade.
I'm not going to by a different brand to save money. Not because I'm worried about quality, but when you're doing some one off project that you need a different attachment or accessory for, its easier to find Dremel brand/compatible gear from friends than XBrand accessories.