Unable to use all cores with mpirun
Use $ lscpu the number of cores per socket * number of sockets would give you number of physical cores(the ones that you can use for mpi) where as number of cores per socket * number of sockets * threads per core will give you number of logical cores(the one that you get by using the command $ cat /proc/cpuinfo | grep processor | wc -l)
Your processor has 4 hyperthreads but only 2 cores (see the specs here).
By default, Open MPI does not run more than one MPI task per core. You can have Open MPI run up to one MPI task per hyperthread with the following option
mpirun --use-hwthread-cpus ...
FWIW
The command you mentioned reports the number of hyperthreads.
A better way to figure out the topology of a machine is via the lstopo command from the hwloc package.
MPI tasks are not bound on cores nor threads on OS X, so if you are running on a Mac, the --oversubscribe -np 4 would lead to the same result.
To resolve your problem, you can use the --use-hwthread-cpus command line arguments for mpirun, as already pointed out by Gilles Gouaillardet. In this case, Open MPI will treat the thread provided by hyperthreading as the Open MPI processor. Otherwise, it will treat a CPU core as an Open MPI processor, which is the default behavior. When using --use-hwthread-cpus, it will correctly determine the total number of processors available to you, that is, all processors available on all hosts specified in the Open MPI host file. Therefore, you do not need to specify the "-n" parameter. In addition, when using the --use-hwthread-cpus command line parameter, Open MPI refers to the threads provided by hyperthreading as "hardware threads". With this technique, you will not oversubscribe, and if some Open MPI processor will run on a virtual machine, it will use the correct number of threads assigned to that virtual machine. And if your processor has more than two threads per core, as a Xeon Phi (Knights Mill, Knights Landing, etc.), it will take all four threads per core as an Open MPI processor.