The functionality that is available in Fluint 1.1.1 is also available inside of FlexUnit 4. However, Fluint 1.1.1 is a release version. Right now FlexUnit 4 is just in beta, so you need to keep that in mind.
They are completely code compatible though, so anything written in Fluint 1.1.1 will work in FlexUnit 4 without change. FlexUnit 4 offers a new metadata model for marking tests and case and will continue to offer a lot of additional functionality.
So, depending on your schedule, etc. you will need to decide what is best for your project.