The siliqua derived it's name for the Latin (Roman) the carob seed, six of which made up the weight of scruple (1/1728 of a Roman Pound).
The gold semissis (1/2 solidus) was valued at 12 silver siliquae.
The gold 1 1/2 Scripulum was valued at 9 silver siliquae
The gold tremissis (1/3 solidus) was valued at 8 silver siliquae.
I cannot find any evidence that a gold 1/4 solidus, that would have been valued at 6 siliquae, was ever minted.
David Sear has presented an interesting read on this subject: ( Roman Coins and their Values - 1988) under the coinage reforms of Doicletian, Constantine the Great, and Constantius 11.