All of our tests run MongoDB in FCV 4.2, with the exception of our multiversion tests, which test predictable update/downgrade paths. We have a notable lack of comprehensive regression testing for when a binary is in FCV 4.0.

One possibility would be to run the compatible jstests from our 4.0 branch against a 4.2 mongod in FCV 4.0 to observe regressions.

Another option could simply be a passthrough or fuzzer suite that runs continuous upgrades/downgrades.