Grok 4 Fast Reasoning vs Qwen3.5-4B
Grok 4 Fast Reasoning (2025) and Qwen3.5-4B (2026) are frontier reasoning models from xAI and Alibaba. Grok 4 Fast Reasoning ships a 2M-token context window, while Qwen3.5-4B ships a 262K-token context window. This comparison covers specs, pricing, capabilities, benchmarks, provider availability, and production fit. It focuses on practical selection signals rather than broad model-family marketing. The goal is to make the tradeoff clear before deeper testing.
Grok 4 Fast Reasoning fits 8x more tokens; pick it for long-context work and Qwen3.5-4B for tighter calls.
Decision scorecard
Local evidence first| Signal | Grok 4 Fast Reasoning | Qwen3.5-4B |
|---|---|---|
| Decision fit | Long context and Vision | Long context and Vision |
| Context window | 2M | 262K |
| Cheapest output | $2.5/1M tokens | - |
| Provider routes | 2 tracked | 0 tracked |
| Shared benchmarks | 0 rows | 0 rows |
Decision tradeoffs
- Grok 4 Fast Reasoning has the larger context window for long prompts, retrieval packs, or transcript analysis.
- Grok 4 Fast Reasoning has broader tracked provider coverage for fallback and procurement flexibility.
- Grok 4 Fast Reasoning uniquely exposes Reasoning in local model data.
- Local decision data tags Grok 4 Fast Reasoning for Long context and Vision.
- Local decision data tags Qwen3.5-4B for Long context and Vision.
Monthly cost at traffic
Estimate token spend from the cheapest tracked input and output prices on this page.
Grok 4 Fast Reasoning
$1,625
Cheapest tracked route: xAI Console
Qwen3.5-4B
Unavailable
No complete token price in local provider data
Cost delta unavailable until both models have sourced input and output token prices.
Switch friction
- No overlapping tracked provider route is sourced for Grok 4 Fast Reasoning and Qwen3.5-4B; plan for SDK, billing, or endpoint changes.
- Check replacement coverage for Reasoning before moving production traffic.
- No overlapping tracked provider route is sourced for Qwen3.5-4B and Grok 4 Fast Reasoning; plan for SDK, billing, or endpoint changes.
- Grok 4 Fast Reasoning adds Reasoning in local capability data.
Specs
| Specification | ||
|---|---|---|
| Released | 2025-09-01 | 2026-03-02 |
| Context window | 2M | 262K |
| Parameters | — | 4B |
| Architecture | decoder only | - |
| License | Proprietary | Apache 2.0 |
| Knowledge cutoff | - | - |
Pricing and availability
| Pricing attribute | Grok 4 Fast Reasoning | Qwen3.5-4B |
|---|---|---|
| Input price | $1.25/1M tokens | - |
| Output price | $2.5/1M tokens | - |
| Providers | - |
Capabilities
| Capability | Grok 4 Fast Reasoning | Qwen3.5-4B |
|---|---|---|
| Vision | Yes | Yes |
| Multimodal | Yes | Yes |
| Reasoning | Yes | No |
| Function calling | No | No |
| Tool use | No | No |
| Structured outputs | No | No |
| Code execution | No | No |
Benchmarks
No shared benchmark rows are currently sourced for this pair.
Deep dive
The capability footprint differs most on reasoning mode: Grok 4 Fast Reasoning. Both models share vision and multimodal input, so the practical split is not just feature count. Use those differences to decide whether the page is about raw model quality, agentic coding support, multimodal ingestion, or predictable structured API behavior.
Pricing coverage is uneven: Grok 4 Fast Reasoning has $1.25/1M input tokens and Qwen3.5-4B has no token price sourced yet. Provider availability is 2 tracked routes versus 0. Treat unknown pricing as an integration gap, then verify the route you will actually call before estimating production spend.
Choose Grok 4 Fast Reasoning when reasoning depth, larger context windows, and broader provider choice are central to the workload. Choose Qwen3.5-4B when vision-heavy evaluation are more important. For production, rerun your own prompts through the exact provider, region, and tool stack you plan to ship. This keeps the decision grounded in measurable tradeoffs instead of brand-level assumptions. It also helps separate model capability from provider packaging, which can change cost and latency. For teams standardizing a stack, that distinction is often the difference between a benchmark win and a reliable deployment.
FAQ
Which has a larger context window, Grok 4 Fast Reasoning or Qwen3.5-4B?
Grok 4 Fast Reasoning supports 2M tokens, while Qwen3.5-4B supports 262K tokens. That gap matters most for long documents, large codebases, retrieval-heavy agents, and conversations where earlier context must remain visible.
Is Grok 4 Fast Reasoning or Qwen3.5-4B open source?
Grok 4 Fast Reasoning is listed under Proprietary. Qwen3.5-4B is listed under Apache 2.0. License labels affect whether you can self-host, redistribute weights, or rely only on hosted APIs, so confirm the upstream license before deployment.
Which is better for vision, Grok 4 Fast Reasoning or Qwen3.5-4B?
Both Grok 4 Fast Reasoning and Qwen3.5-4B expose vision. The better choice depends on benchmark fit, context budget, pricing, and whether your provider route exposes the same capability surface. Use this as a quick comparison signal, then confirm the provider-specific limits before committing to production.
Which is better for multimodal input, Grok 4 Fast Reasoning or Qwen3.5-4B?
Both Grok 4 Fast Reasoning and Qwen3.5-4B expose multimodal input. The better choice depends on benchmark fit, context budget, pricing, and whether your provider route exposes the same capability surface.
Which is better for reasoning mode, Grok 4 Fast Reasoning or Qwen3.5-4B?
Grok 4 Fast Reasoning has the clearer documented reasoning mode signal in this comparison. If reasoning mode is mission-critical, validate it against the provider endpoint because model-level support and API-level exposure can differ.
Where can I run Grok 4 Fast Reasoning and Qwen3.5-4B?
Grok 4 Fast Reasoning is available on Microsoft Foundry and xAI Console. Qwen3.5-4B is available on the tracked providers still being sourced. Provider coverage can affect latency, region availability, compliance posture, and fallback options.
Continue comparing
Last reviewed: 2026-05-20. Data sourced from public model cards and provider documentation.