At this point, the four boys you mention are pretty much all that is left of the civilized group. They have been attacked and beaten by the savages. The reason they want to go up to Castle Rock is to try to get some control back -- they want to go tell the savages that they need to behave properly. It is like a last gasp effort to restore sanity to the island. Look at Piggy's speech about 4 pages in to the chapter -- the one about what he is going to tell Jack. That pretty much sums up why they are going up there.
Piggy takes the conch, the other three have spears.