Updating Craft 3 from RC3 to RC4 through Composer
So, not all hosts allow just using composer and impose to use php composer.phar.

Knowing that, I can now answer my two questions:


When installing Craft using php composer.phar create-project -s RC craftcms/craft $HOME/craft, the -s switch is just telling which branch of the source project to use. Here, RC stands for Release Candidate. Before that, we had to use -s BETA in order to install the betas. Something I didn't knew since I waited for the RC to jump in Craft 3 and start over from Craft 2.
When you want to update using php composer.Phar, you need to go inside your project folder (where you installed Craft; craft for me which means once logged I had to cd craft) and then type php ../composer.phar update, where ../composer.phar tells to go looking for Composer from the parent folder.


I am ashamed not having thought of that; I really need to stop overthinking things.