v4.0.0-beta.4

Changes

• Fix NavLink location variable (#4403 by @inuscript)
• NavLink tests (#4421 by @pshrmn)
• Preact support (#4423 by @kwelch)
• Allow initialIndex, initialIndex in NativeRouter (#4415 by @iljadaderko)
• Add <NavLink strict> and <NavLink exact>
• invariant dependency was missing