Xcode 4.0 Release was released on 9 Mar 2011 and requires macOS 10.6.8+.
It can be downloaded from https://download.developer.apple.com/Developer_Tools/xcode_4_and_ios_sdk_4.3__final/xcode_4_and_ios_sdk_4.3__final.dmg.
Release notes are located at https://download.developer.apple.com/Developer_Tools/xcode_4_and_ios_sdk_4.3__final/xcode_4_rn.pdf.
The compilers it contains are:
- clang 2.0 (Build 137)
- LLVM-GCC (Build 2335.9)
- GCC 4.0 (Build 5494)
- GCC 4.2 (Build 5666)