I'm looking for a small 2-4 channel field mixer that has the ability to output via optical audio. Something like the Promix 3 or the MX422, but with an optical out.

Does anyone know of such a product?

Just out of curiosity, why would you need an optical output?
What is your workflow?

I'm looking for a small 2-4 channel field mixer that has the ability to output via optical audio. Something like the Promix 3 or the MX422, but with an optical out.

Does anyone know of such a product?

The only portable mixer w/ optical out I'm aware of is the Roland M-10MX...but it looks weak on the mic pre's.

http://www.rolandus.com/products/productdetails.aspx?ObjectId=862

The Sony DMX-P01 (http://www.sonybiz.net/biz/view/ShowProduct.action?product=DMX-P01&site=biz_en_BE&pageType=Overview&imageType=Main&category=Mixers) is equipped with a digital output, which can be used to send audio to digital peripheral equipment such as DAT recorders.
AES/EBU and S/PDIF (IEC 60958) coaxial interfaces are available for master outputs.

That sony is the only digital field mixer that I have seen, and is definitely not at all in the price range of either of the mixers you mentioned. No mic pre's on that roland, but the next one up, the mx-10dx, might fit the bill (the unit has two pre-amps), but running off battery power will be expensive. If your recording 2-track any way, you might want to look into an Apogee mini-pre. A little more expensive and not a mixer, but a solid stereo pre with converter and optical out.