What would happen if a nucleotide was added at the fourth location in the chain

A frame shift mutation will occur.

This the type of gene mutation arises when the normal sequence of codons is disrupted by the insertion or deletion of one or more nucleotides, provided the number of nucleotides is added or removed is not a multiple of three. For instance, if just one nucleotide is added, then all of the codons including and after the mutation will have disrupted reading frame. This can result in the inclusion of many incorrect amino acids into the protein.