Cannot set MEMORY_TARGET for Oracle 11.2
First look what other parameters are set:

CREATE pfile='pfile.txt' FROM spfile;


Maybe db_cache_size, shared_pool_size, sga_target or other memory related parameters are set to non zero? Remember that when using AMM those parameters specify minimum memory allocated for particular pool. So if sga_target is 6GB you will not be allowed to set memory_target to 4GB.

Also sum of internal variables __sga_target, __db_cache_size, etc. may be more than your specified value of 4GB.

If you see those symptoms you can cleanup pfile bounce Oracle with pfile and recreate spfile. In the same step you can also set PGA_AGGREGATE_TARGET to zero.

STARTUP pfile='pfile.txt'
CREATE spfile from pfile='pfile.txt';