I presume that CPU_HOTPLUG means hotplug cpu governor is enabled in your system.
hotplug cpu governor can make cpu core online or offline based on total system load.
So, It might be possible that your other two cores are offline at the moment.
You can check the exact cpu cores by,
$ cat /sys/devices/system/cpu/present
This will display total cores on your system. Or you can do "ls /sys/devices/system/cpu/cpu*" then see cpu0,cpu1,... directories. Those are your different cores.
If you can not find 4 cores (0,1,2,3) in that directory, than there is some problem with your system.
You can change the cpu governor to some other for example, ondemand to utilize all four cores. But it may use more battery.
I hope this helps.
Also, if you are referring to set the default kernel command line from configuration then it is available at,
Symbol: CMDLINE [=]
Type : string
Prompt: Default kernel command string
Defined at arch/arm/Kconfig:1766
-> Boot options
in kernel menuconfig window.
Generally in android, kernel commandline is part of boot image and not this default one, But you can still try that out. If bootloader is passing the kernel command line string than this configuration will be ignored.