They're only going to build and ship 350 T4Rs to the entire US market per month. They're focusing production on more popular models like the Highlander and Sienna.
That doesn't make any sense to me. Highlander and Sienna are built in the US. 4Runner is built in Japan. So reducing the build for the 4Runner has no impact on the number of Highlanders and Siennas that they can build.
Now it won't surprise me if Toyota decides to build fewer 4Runners. But I don't think it has anything to do with production numbers for Highlanders and Siennas.