The dicarbon molecule, C₂, does exist in nature. It has been detected in stellar atmospheres, the interstellar medium, and cometary comas. The fluorescence of C₂ in the latter contributes to the green appearance of certain comets, including C/2022 E3 (ZTF) currently approaching Earth.

However as discussed in this paper (link to abstract only), the 4th inter-carbon bond does not make for a 'stronger' molecule:

>Quantum chemical calculations using the complete active space of the valence orbitals have been carried out for HnCCHn (n=0–3)...

>The bond dissociation energies and the force constants suggest that C₂ has a weaker C−C bond than acetylene, C₂H₂.