ncu v3 excluded prerelease versions (-alpha, -beta, etc) from the remote by default, as publishing prerelease versions to latest is unconventional and not recommended. Prereleases versions can be included by specifying --pre (and is implied in options --greatest and --newest).
However, when you are already specifying a prerelease version in your package.json dependencies, then clearly you want ncu to find newer prerelease versions. This is now default in v4, albeit with the conservative approach of sticking to the latest tag.
Migration
No effect for most users.
If a prerelease version is published on the latest tag, and you specify a prerelease version in your package.json, ncu will now suggest upgrades for it.
If a prerelease version is published on a different tag, there is no change from ncu v3; you will still need --pre, --greatest, or --newest to get prerelease upgrades.