How do Jack and Ralph end up (on different sides of a high barrier) literally and metaphorically in this chapter -4(73)? How is a link between Ralph and Jack snapped and fastened elsewhere (73)?

The high barrier refers to the pile they had built for the fire. Ralph and Jack are on different sides of it. The metaphorical interpretation is much more interesting. By chapter four, both boys are in separate worlds. Jack wants to hunt and wield power while Ralph wants to be rescued and, until rescue, have some form of a fair functioning society. 

By the time the pile [fire] was built, they were on different sides of a high barrier....Not even Ralph knew how a link between him and Jack had been snapped and fastened elsewhere.