The update speed of the chart will depend on a large number of factors, including the amount of data being shown (though sounds like this is modest in your case), the hardware you're running on, what sorts of axes you're using, how many tick marks and labels are showing, how many other elements there are (titles, legend, annotations, etc), the complexity of the styling applied and a number of other variables. However, you aren't going to be able to get a chart that redraws on a 4ms frequency as that would equate to about 250 frames per second. The ipad/iphone updates at a screen refresh rate of 60Hz, which is a limitation of the device/OS and so you aren't going ot be able to go faster than that (which is about one update per 16.66ms). There is considerable work and delay, however, in transferring data onto the GPU hardware so it's unlikely you'll be able to achieve that speed on a reload of data (though you should be seeing something close to 60fps for panning and zooming operations as the data is already in place and most of the work is done on the GPU).
I have managed to replicate your scenario of 128 columns and I turned on background data loading and was seeing about 30fps on an ipad 3. Clearly, I don't know much about your specific use cases but I think it should be possible for you to hit anywhere between 20 and 30 fps unless you're working with large amounts of data or complex data processing scenarios. Note that this applies only when you are loading new data. The chart can certainly redraw and/or pan and zoom at a much higher frequency, all the way up to the max 60fps.
While you can certainly accept data every 4ms, on iOS you won't be able to get your screen refreshing faster than once every 16ms at the very best. I hope that's useful information.