The closest alternative to Claude Sonnet 4 is o4 Mini — comparable intelligence (33.1 vs 33.0); for a cheaper option, GPT-5.4 runs $2.50/M vs $3.00/M; for more speed, gpt-oss-120b (free) hits 554 t/s; and the best open-weight substitute is MiniMax M3.
o4 Mini is the closest alternative to Claude Sonnet 4, with an Intelligence Index of 33.1 (Claude Sonnet 4 scores 33.0) at $1.10 per million input tokens.
Yes — GPT-5.4 costs $2.50 per million input tokens versus $3.00 for Claude Sonnet 4, while staying within ~8 points of its Intelligence Index.
MiniMax M3 is the strongest open-weight alternative to Claude Sonnet 4 — downloadable from Hugging Face and self-hostable, scoring 54.7 on intelligence.