  Most co-orbital objects in the Solar system are thought to follow
tadpole-type orbits, behaving as Trojans. However, most of Earth's identified
co-orbitals are moving along horseshoe-type orbits. The current tally of minor
bodies considered to be Earth co-orbitals amounts to 18; of them, 12 are
horseshoes, five are quasi-satellites, and one is a Trojan. The semimajor axis
values of all these bodies librate between $0.983$ au and $1.017$ au. In this
work, we have studied the dynamical behaviour of objects following orbits with
semimajor axis within this range that may be in a 1:1 mean-motion resonance
with Earth. Our results show that asteroids 2016 CO$_{246}$, 2017 SL$_{16}$,
and 2017 XQ$_{60}$ are moving along asymmetrical horseshoe-type orbits; the
asteroid 2018 PN$_{22}$ follows a nearly symmetric or regular horseshoe-type
orbit. Asteroids 2016 CO$_{246}$, 2017 SL$_{16}$, and 2017 XQ$_{60}$ can remain
in the horseshoe co-orbital state for about $900$ yr, $3300$ yr and $2700$ yr,
respectively. Asteroid 2018 PN$_{22}$ has a more chaotic dynamical behaviour;
it may not stay in a horseshoe co-orbital state for more than 200 yr. The
horseshoe libration periods of 2016 CO$_{246}$, 2017 SL$_{16}$, 2017 XQ$_{60}$,
and 2018 PN$_{22}$ are 280, 255, 411, and 125 yr, respectively.
