If you look at Google's API pricing, Gemini 4 Carbon and Gemini 4 Argon are the same while Gemini 4 Hydrogen and Gemini 3.8 Flash are the same (ignoring promotional pricing).

Intuitively, Argon is 4 Pro, Carbon is 4.1 Pro, and Hydrogen is 4.1 Flash (slightly better than Argon according to benchmarks).