Current JDK7 gcc version is 3.2.1-7. Among the benefits of gcc4 are -fvisibility=hidden option, allowing better DSO load time, better code, size reduction (see also 6588413); as well as better memcpy performance. gcc4 should also be more community-friendly as many external contributors expect gcc4 builds to work by default.

Among other things, we'd need to make sure that gcc4-built binaries can run on the oldest of our JDK7 target platforms, as well as extensive testing to flush out new compiler bugs, performance regressions, etc.