Instruments New Features User Guide summarizes some of the new features in Instruments 4.
At a Glance
Instruments 4 is the latest iteration of Apple’s application for dynamically tracing and profiling iOS and OS X code. Instruments is available with Xcode, the complete toolset for building iOS and Mac apps.
To get the most out of reading this document, you should be familiar with the basic features in earlier versions of Instruments, described in Instruments User Guide.