OK. Probably a silly question, but have you actually installed Ruby 2.4?
You will see a list of installed Ruby versions.
=> to the left of a version indicates the current version;
* indicates the default version;
=* indicates the current && default version.
If Ruby 2.4 does not appear in the list, install it with
rvm install ruby 2.4
I imagine that would solve the problem. If, however, it is in the list of installed versions but not set as current and default version, enter
rvm --default use 2.4
I’m not sure the last command will be of use, but it’s there in case you need it.
If that doesn’t help, I’ll have to leave it to someone else.