It’s not 4 screens, it’s 3 devices! (although he doesn’t mention cinema)

handheld device – useful for audio communication or small messages
active device – the personal computer
entertainment device – big screen TV

They fulfil different functions but are complementary.

Transcript:
There are 3 fundamental types of devices that people use and want and I think will continue to exist. They are the handheld device which is useful for audio communication or small messages. There is the active device which is the personal computer. And there is the entertainment device which is the consumer electronics, big screen TV. Those devices occupy separate spaces, they fulfil different functions. They should be, and are increasingly going to be, complementary. You can get the Internet on you big screen TV. You can do all sorts of video conferencing or messaging with your PC as well as interactive content creation. And your handheld device is increasingly an Internet device. I don’t see really challenges between them. I see them as complementary and bringing capability to the end consumer. You know, how many of us are going to give away our PC and try to write a text on our cell phone? How many of us are going to give away our TV and watch TV on a PC? They are complementary devices and I don’t think we should confuse them by saying one is going to dominate. They all fulfil separate functions.
[…]
We are going to have our personal devices and those are the ones we carry with us all the time. And they allow us access to information and communication. We are going to have those devices that allow us to create and deal with rich content, that’s the PC. And then we are going to have those devices that basically entertain us, the big screen television. Now, they will interact with each other and we will be able to move information from one to the other and that will be one of the beauties. They will be stove piped, they won’t be isolated applications, but we will be able to move things from our PC to our television set to our handheld device and vice versa. So, I think that this is the way for the future. You will carry devices with you. You will have devices which you will use routinely in your work to create content, to create innovation. And then we will always want the entertainment part of it.

BBC World Service ~ Global Business: Craig Barrett (19.06.2009)