people use all T N S F, latter functions are unconscious by definition. thats because your first function contradicts the fourth, thus repressing it to unconscious. first is developed first, second is developed to aid the dominant, third is developed from the dialect/conflict between dom and inferior to aid the union of those opposites and to regulate the instincts coming from the unconscious fourth.
and here is couple things that the fourth does:
1) it destroys the certainties and definitions, in that way showing us our limits, especially the limits of our perceptions.
2) it gives the purpose that what earlier seemed as having no purpose. but in order for it to take a specific form and gain the sense of purpose, response to the fourth it required. this is where too large ego(conscious mind) can get in the way, since ego is the opposite of the shadow(unconscious mind, not the 8 function wanna be shadow), which contains the fourth. therefore ego tries to repress the contents of the shadow -> you need to go against the wishes of your ego in order to find a purpose out of the fourth and you wont be able to channel the fourth properly and take it into the concrete life.
by doing these, you are able to conscious of third and fourth functions, even tho you arent able to consciously control them, you are able to consciously control your reactions to them(how well you are able, depends how well you have learned to do so), this means when to stop and when to keep on following it.