The Sigma was a diamond cantilevered model that predated the development of the UNIverse. It's rarely made or sold any more because the UNIverse outplays it (according to Nakatsuka-San) while being less expensive.
The 4D (I'm unaware of any other "4" model) is the same cartridge as the Atmos. Atmos is sold in the USA by SoraSound, 4D is sold in other markets by dealers who purchase from other distributors. There's no technical difference, just the name for marketing purposes.
The 4D/Atmos not the same as the UNIverse. Read the reviews of both cartridges beneath my signature.
The 4D (I'm unaware of any other "4" model) is the same cartridge as the Atmos. Atmos is sold in the USA by SoraSound, 4D is sold in other markets by dealers who purchase from other distributors. There's no technical difference, just the name for marketing purposes.
The 4D/Atmos not the same as the UNIverse. Read the reviews of both cartridges beneath my signature.