Chaturamukha linga or the four faced linga: typically a linga is devided into three parts - the rudra bhaga which is the bottom most part, most often this is not visible as it is placed in the groove of the yoni. the second part is the vishnu bhaga and the top most part is the shiva bhaga. only 3/4ths of this linga will be visible above the yoni. come to think of it shaivism pretty much deals with fertility worship.
More on shiva: The mythology of Shiva is complex and contradictory. He is sometimes depicted as a chaste, wandering ascetic, but at the same time an ideal family man, monogamous, powerful and fertile. Shiva is usually worshipped in the form of the linga, an erect phallus. Rather than an image charged with sexuality, the linga is regarded more as a symbol of the god's energy and potential.
The four faces, each pointing in a cardinal direction, allude to Shiva's most prominent aspects. These comprise Bhairava, a manifestation of Shiva in his terrifying form, two faces which show his different attitudes as a withdrawn and serene ascetic and lastly, Parvati, his consort, the embodiment of feminine power.
The four-faced linga is sometimes regarded as symbolic of the five elements – Earth, Water, Wind, Fire and Space, the latter alluded to by the invisible face imagined to be on top of the sculpture and referred to by the vertical axis.