If I interpret [this comment|https://github.com/mongodb/mongo/blob/dd15f9c211f5bc40b6fa8a7cd44350d3aeb87d89/src/mongo/db/wire_version.h#L59} correctly:

enum WireVersion {
    // ... <snip> ...
    // Support find and getMore commands, as well as OP_COMMAND in mongod (but not mongos).
    FIND_COMMAND = 4,
};

... then we can use wire version to know whether find and getMore commands will succeed, but we can't use wire version to know whether OP_COMMAND will succeed. Instead, we must do something like

if maxWireVersion >= 4 and "msg: isdbgrid" not in ismaster response:
    use OP_COMMAND

That seems needlessly inelegant. Let's bump maxWireVersion to 5 if and only if OP_COMMAND is supported.