Qwen3.5-4B-OptiQ-4bit

Mixed-precision quantized with OptiQ — sensitivity-driven quantization for Apple Silicon

This is a mixed-precision quantized version of Qwen/Qwen3.5-4B in MLX format. Unlike uniform quantization (all layers at the same bit-width), OptiQ measures each layer's sensitivity and assigns optimal per-layer bit-widths, preserving model quality where it matters most.

How OptiQ Works

OptiQ is an optimizing compiler that converts PyTorch models into hardware-optimized MLX versions using data-driven mixed-precision quantization:

1. Sensitivity Analysis — For each layer, OptiQ simulates quantization at each candidate bit-width and measures the KL divergence between the original and quantized output distributions. Layers that distort the distribution more are "sensitive."
2. Greedy Knapsack Optimization — Starting with all layers at the minimum bit-width, OptiQ greedily upgrades the most sensitive layers to higher precision until the target bits-per-weight budget is exhausted.
3. Per-Layer Bit Allocation — The result is a custom quantization config where each layer gets the bit-width that maximizes quality within the size budget. Protected layers (embeddings, final layers) are always assigned high precision.

Quantization Details

Property	Value
Target BPW	4.5
Achieved BPW	4.50
Candidate bits	4, 8
Layers at 4-bit	162
Layers at 8-bit	87
Total quantized layers	249
Group size	64
Model size	2811 MB
Uniform 4-bit size	2258 MB
Calibration data	WikiText-2 (2 samples, 128 tokens)

Benchmark Results

GSM8K (200 samples, 3-shot chain-of-thought):

Model	GSM8K Accuracy
OptiQ mixed (4.5 BPW)	81.5%
Uniform 4-bit	79.5%

OptiQ improves over uniform 4-bit by +2.0 percentage points.

Usage

This model works with standard mlx-lm — no special code needed:

from mlx_lm import load, generate

model, tokenizer = load("mlx-community/Qwen3.5-4B-OptiQ-4bit")

prompt = "Explain quantum computing in simple terms:"
response = generate(model, tokenizer, prompt=prompt, max_tokens=200)
print(response)

Requirements: mlx-lm >= 0.30.7 (for Qwen3.5 architecture support)

pip install mlx-lm>=0.30.7

Architecture

Qwen3.5 uses a hybrid attention architecture with alternating linear_attn and self_attn layers. OptiQ's sensitivity analysis identifies which layers are most sensitive to quantization error and assigns them higher precision, while less sensitive layers get 4-bit quantization to minimize model size.

Article

For more details on the methodology and results, see: Not All Layers Are Equal: Mixed-Precision Quantization for Weights and KV Cache on Apple Silicon

Credits

• Quantization method: OptiQ — optimizing compiler for mixed-precision quantization on Apple Silicon
• Base model: Qwen/Qwen3.5-4B by Qwen Team
• Runtime: MLX by Apple