Both of the processions are similar in that they help us understand how tightly bound this community is, both in sorrow and in joy. Note what we are told in Chapter 4 about the attitude of the neighbours:
The neighbours, watching Kino's door through the crevices in their brush houses, were dressed and ready too. There was no self-consciousness about their joining Kino and Juana to go pearl selling. It was expected, it was an historic moment, they would be crazy if they didn't go. It would be almost a sign of unfriendship.
Clearly, this is a community which is very communal - to not share the joys and the despairs of each others' lives would be a sign of superiority.
The central difference between the two processions is that in Chapter 4 it is a procession of joy as Kino goes to sell the pearl, and Chapter 1 is a procession of mourning as they take their son to the doctor after being stung by a scorpion.
Both processions are described as being solemn, with everyone sensing the importance of what is going on. Interestingly, shadows are both mentioned in the description of either procession to. But the crucial similarity is what is shows about the community of this tiny village.