Oracle Database 12c: Unable to resize system tablespace
DBA_TABLESPACE_USAGE_METRICS shows units in database blocks, not bytes. 4 MB is not enough for a SYSTEM tablespace. Most likely what you saw was 4M blocks, which is 32 GB with a 8K blocksize tablespace, and that is the maximum size of a datafile in a 8K smallfile tablespace. That is why your attempts had no effect.

You can add a new datafile to the tablespace, for example:

alter tablespace system add datafile '/home/user/oracle/orcl/system02.dbf' size 10m autoextend on next 10M maxsize unlimited;