Runtime compatibility is guaranteed only for patch versions (where version numbers are major.minor.patch), so 4.2 runtimes can't officially load 4.1 data. In practice sometimes JSON data from older versions is OK (though not officially supported), but not binary data. The differences between 3.8 and 4.0 are very large, so old JSON data isn't compatible.
Even though minor versions aren't compatible, I don't think it changes the approach. You could still use multiple runtime versions. If you don't want to bloat your codebase with many runtime versions, you don't need to update to every new major.minor version. Maybe just once/year, or maybe just do it once, so your animators have access to 4.0. In that case you may want to wait for 4.1, which improves on the graph and other new features added in 4.0.
