Update RxUI to use System.Reactive >= 4

[glennawatson]

Do you want to request a feature or report a bug?
Upgrade

What is the current behavior?
Referencing System.Reactive 3.1.1

What is the expected behavior?
Update to System.Reactive >= 4.0.0

This avoids having NU1608 come up when compiling.
warning NU1608: Detected package version outside of dependency constraint: ReactiveUI 8.2.1 requires System.Reactive (>= 3.1.1 && < 4.0.0) but version System.Reactive 4.0.0 was resolved.

What is the motivation / use case for changing the behavior?
RxUI is the main consumer of Rx and should maintain version parity.

Which versions of ReactiveUI, and which platform / OS are affected by this issue?
8.2.1, all

[ghuntley]

Fixed by #1674. Shipping 8.3.x to nuget now which contains this.

[glennawatson]

Users are reporting weird issues which are solved by updating to 4.0.0