There are two primary things you can do to reduce power consumption:
Minimize the amount of power consuming peripherals attached and running.
Minimize the workload. A major consumer of power is the CPU. When it is idle, it consumes less power.
That's it. The pi is a low power device compared to regular computers, but not so low power compared to many mobile devices such as smart phones. Unfortunately, there is not really much that can be done about that without adding more hardware.
What is the correct command to put the raspberry pi to sleep?
There isn't one because the hardware does not have ACPI or any other kind of power management. It is either plugged in, in which case it is on, or not.
There are commands you can issue that would work if the device did have ACPI, but it doesn't, so they won't.
Any hardware hacks that allows to control the raspberry pi power management?
No, since again, it does not have any power management.
Would it be possible to disable the remaining 3 usb ports only?
I'm not sure that doing this in the first place would really make any difference. It is like saying you would like to disconnect some empty plugs in your house to save power. If there is nothing plugged into them, they weren't using any in the first place.
Technically that's not quite true -- the USB bus itself is a microcontroller and so uses power (probably a minuscule amount when idle). However, I'm not sure that the normal recommended method for disabling USB power:
echo 0 > /sys/devices/platform/bcm2708_usb/buspower
Actually turns off the bus completely; more likely it just prevents USB devices from drawing power from the bus. Put another way, I've seen various people claiming they do this to save power, but the only evidence I've seen involves a LAN connection (the ethernet jack is on the same bus) and does not say how much simply unplugging the ethernet cable would have saved.
In any case, all the ports are on one bus, so if you disable that, you disable them all.