Multiple checks exist to ensure that the right FCV is being used. Once the major version 4.0 releases, we can remove these checks. As of 4/3/2018, these files use FCV logic checks:

configsvr_move_primary_command.cpp
sharding_catalog_manager_database_operations.cpp
move_primary_source_manager.cpp

Additionally, these tests check both the FCV 3.6 and FCV 4.0 code paths. Once the FCV 3.6 code is removed, the FCV 3.6 portions of these tests can be removed as well.

moveprimary-replset.js
movePrimary1.js
move_primary_basic.js
move_primary_clone_test.js

Also consider a rename (or deletion) of move_primary_fcv_40.js, without losing the specific DatabaseVersion checks (add them somewhere else, maybe).

As a last look, consider deleting _buildMoveLogEntry in configsvr_move_primary_command. We may not need it after everything else is gone.