4890 is a revision of the 4870. It runs cooler and is clocked higher, and came out about 6 months after the 4870. If DirectX11 is important to you (vista/7 only) than a 5770 might be up your alley, it performs very similar to a 4870, and uses a lot less power than either of them, as well as should be cheaper.
Normally, the 5830 is not an easy card to recommend, as it uses more power than a 5850 (because it is a clipped 5870) and performs up to or better than a 4890. BUT, the prices have come down on them, so for a hard price cap, the 5830 is probably the best single card you can buy.
looking for a new video card and was thinking between these 2. What is the difference between the 4870 and 4890. Or, any other suggestions. I would like to stay at or under $200.
Keep in mind that for under 200$, the choice is great. The biggest issue that you want to clear right off the bat is whether having a DX11 capable card is important to you. The DX11 ATI cards (5000 series) have the benefits of being less power hungry, cooler (and probably quieter), support eyefinity and obviously come with DirectX11 support. The older 4000 series use more power, are hotter, do not support DX11, but they cost much less, since they're at the end of their practical sell life. See it like buying a car from last year.
However, also keep in mind that while the newer cards have all those benefits, they have about the same raw power as the older ones, so you won't get a clean FPS improvement over them.
That said, here are just a few of the options available to you:
You probably will have a lot of trouble finding a 4890, and from what I see, the 4870 is priced about the same as a 5770 (which has about the same raw power, but all the newer features), so you'd have no reason to go with a 4870. Here's what this might look like: http://www.newegg.com/Product/Product.aspx?Item=N82E168...