If you're determined to download 400-in-1 NES ROMs, doing so safely and ethically is paramount. Here are some guidelines:

: Primarily found on handheld consoles that include a built-in screen, rechargeable battery, and TV-out capabilities for playing on larger displays. Purchasing Options

While the list claims 400 unique titles, it often includes several repeats or hacks. Common classics found in these sets include: Sup 400-in-1 ROM dump analysis (VT-XX system?)

Since these multicarts are unofficial "bootlegs," they are primarily hosted on community preservation and ROM archival sites: Internet Archive (NES Mega Pack)