thank you for writing in, and for evaluating CrateDB in the context of Jupyter Notebooks and Spark, which also sparks [sic!] my interest. I can look into further details of this topic next week.
In general, to second @hammerhead, it is recommended to use the vanilla PostgreSQL JDBC driver [1] with CrateDB. Also in general, when aiming to connect to the PostgreSQL-compatible interface of CrateDB, addressing it on port 4200 is probably wrong, because this is the standard port of its HTTP interface.
It will probably not improve anything on your error, because it looks like the application is not even connecting to CrateDB, but croaks when loading the driver already. Still, I wanted to make you aware of the details I’ve spotted within your original post.
Please let us know about the outcome when using the vanilla driver, where the correct driver class name is org.postgresql.Driver.