Question about Arctic Mx-4

Comes between CPU and heat sink ( if the latter doesn't already have something on it ), so technically yes.

Depending on the size of the heatspreader (the metal protective cap on the CPU under which the DIE (the actual processor) is located), you will need between a grain of rice and a pea's worth of paste.

Apply to the centre of the HS, install the heat sink and remove it again and check that the entire HS has been wetted by the contact pressure - if yes, install the heat sink for good. If not, apply one more grain of rice.

Where else?