This is the second release candidate for version 1.4. The only change is the removal of lingering conditional checks at compile and run time for GDAL versions < 3.5, across all project modules. For example, in rasterio.dtypes we no longer check for GDAL support of 64-bit integers and work around their absence, we presume that support for 64-bit integers exist.
If you use Rasterio 1.4.0 with GDAL versions < 3.5, you may experience unexpected Python exceptions, and, rarely, undefined behavior. Wheels from PyPI and conda-forge distributions, for example, meet this requirement. Please test your deployments and upgrade Rasterio and GDAL together if you are building rasterio from source.