Would the h4n be a viable recording device for a run and gun feature doc shot on 5d and 7d cannon, With 416/rycote/boom and radio lav for 2 channels and the stereo mic for room/street atmos track? Or should I get an over the shoulder fostex mixer and solid state recorder and use the H4n as backup? The specs of the H4n seem good but how good are the pre/amps? It would save a lot of space as the doc runs from Dubai to Palestine
If you can afford a mixer and recorder, definitely go with that! A stand alone mixer will definitely sound better than the Zoom's preamps.
Also, while he H4n is capable of providing acceptable sound, you can't mix with it. You pretty much have to set it and forget it - you can't really get away with riding the gains on a zoom.
Also, I wouldn't use the internal stereo mics for atmos - most of that would be done in post anyways. You could get some wild track of it if you really wanted to (if you can get the crew quiet for long enough!), but in the quietest of situations, the Zoom's internal mics will probably be too noisy anyways.
Just my 2 cents.
Just another quick side note - be careful with the 7D - they like to drift a lot, especially on longer shots (5d does sometimes as well). See if you can convince them to stick with shorter shots!
If you have the budget get a Sound Devices MixPre, it is still pretty small and will not add too much bulk to your travels but it will drastically improve the sound you get out of the H4n.
Take a stroll through ebay, or craiglist and you should be able to find a MixPre used for a pretty good price. Plus they are pretty solidly made, so you can be fairly sure a used one will still be a workhorse.