Are there any other (meta)stable oxygen molecules beyond O3?
According to my German version of Holleman-Wiberg: Inorganic Chemistry there is one additional, very short lived species, the so called tetraoxygen. A short article is also available on Wikipedia.

Wiberg describes it's synthesis from dioxygen in a mass-spectrometer employing the "neutralisation-reionisation-technique" (I am translating and paraphrasing at the same time, there might be something lost in translation.) It's stable time frame is in the micro second area. The decomposition barrier of $\ce{O4}$ is about 25&nbsp;kJ/mol. The structure is not known, but most likely an aggregate of two oxygen molecules, i.e. $\ce{(O2)2}$. In lower concentrations $\ce{O4}$ might be found in liquid oxygen and they may be a template for the  &epsilon;-high pressure modification of solid oxygen forming at 0.6&nbsp;Mbar.


Holleman, A., Wiberg, N., Wiberg, E., et al. (2008). Lehrbuch der Anorganischen Chemie. Berlin, Boston: De Gruyter. Kapitel XIII 1.1.4 Kurzlebige Sauerstoffspezies. S. 513f.  (in german)
Translation of the 101st edition available as Egon Wiberg and Nils Wiberg (2001) Holleman, Wiberg. Inorganic Chemistry. Academic Press.