To answer your question accurately, I would typically need to see the phylogenetic tree you are referencing. However, in general, within a phylogeny, the species that are most closely related are those that share a more recent common ancestor.
If you can describe the phylogenetic tree or the relationships among the species, I could help determine which species is most closely related to species 4. Alternatively, if you have specific characteristics or branch points in mind, please share those details!