Is it possible, in principle, for a second row homonuclear diatomic system to show a bond order of 4 in an excited state?
The normal excited state is one in which a single electron is promoted from a lower orbital into one of the anti-bonding ones. Thus the situation is initially $\cdots g^2$ for 2 electrons in a ground state orbital g (need not be the highest orbital but often is), and, say, after absorbing a photon of the correct energy the situation is $\cdots g^1a^1$ where a is an anti-bonding orbital. This combination of orbitals is called the excited state. (If the electron spin remains paired it is called a singlet excited state, if unpaired ('parallel') it is called a triplet excited state). As one electron is now in an anti-bonding orbital the molecule has overall weaker bonding. (However, the NO excited state may be an exception).

The excited state always decays back to the ground state (it in not a stationary state of the hamiltonian) with a lifetime that depends on the particular molecule or atom and between different molecules the lifetime
can have  a huge range, from less than a picosecond to several seconds.