Xcode 9.3 Beta 4 was released on 5 Mar 2018 and requires macOS 10.13.2+.
It can be downloaded from https://download.developer.apple.com/Developer_Tools/Xcode_9.3_beta_4/Xcode_9.3_beta_4.xip.
The SDKs it contains are:
- iOS 15E5201e
- macOS 17E170c
- tvOS 15L5198d
- watchOS 15T5199e
The compilers it contains are:
- clang 9.1.0 (Build 902.0.37.1)
- swift 4.1 (Build 902.0.43)