SGA_MAX_SIZE and SGA_TARGET
It is perfectly valid to increase the SGA_MAX_SIZE without changing the SGA_TARGET though it probably isn't going to change your performance.  Setting the SGA_MAX_SIZE to a larger value allows you to adjust the SGA_TARGET upward without restarting the database.  It won't affect the actual amount of RAM allocated to the SGA, that's controlled by the SGA_TARGET.  If your intention was to increase the size of the SGA from 2 GB to 4 GB, you need to adjust the SGA_TARGET.