The only safe and supported way to get Nexus 4 is through the official reFX website . The plugin is sold in different tiers based on how many "expansions" (additional sound packs) are included:
Adds 10 additional expansions, bringing the total to over 5,600 presets.
The Truth About " reFX Nexus 4 Free Download" If you are looking for a reFX Nexus 4
While the software itself is paid, you can find free content if you already own a legitimate version: REFX Nexus 4 - Free Preset Pack Review