Get the Iris graphics one. Trust me - the 4000 is a huge advance for intel in graphics but it's much slower than the Iris. If you can afford it, get the iris.
As for videos, it depends on if the encoder is using the GPU. If it's just using the CPU, it won't make much of a difference. If it's using the compute cores on the GPU, then you'd see a big difference. The 4000 does about 332 GFLOPS while the Iris ( without the 128mb cache the Iris Pro has ) does 832 GFLOPS. So it's like twice as fast. And if you're talking about the Iris Pro, not just the Iris, then without a question get the Iris Pro ( that fast 128mb cache really makes a difference )
Check out this page on Wikipedia for more information about the Intel Graphics and the differences between the models:
http://en.wikipedia.org/wiki/Intel_HD_and_Iris_Graphics
Good luck with your new MacBook!