The Zoom Q8 is a camcorder with very good audio features but it will not eliminate background noise.
To cut down on ambient noise, make sure your mic is placed very close to the subject.
1) You can place cabled or wireless lavalier mics on your subjects. Zoom Q8 can support up to 4 tracks of audio. (2 mono XLR and 1 stereo pair via 3.5mm input)
2) You can boom a shotgun mic (for outdoors) or a supercardiod (for indoors) over your subject's head. There are now many cheap wireless 2.4GHz tranciever sets to use with 3.5mm mics. Or go with senheisser SKP 100 wireless XLR if you are using XLR mics.
If you prefer cables, Zoom Q8 can even supply phantom power to your XLR mics. Cables can run through a boom and out of shot into the Zoom Q8.
If you are shooting with GH4/ VG30, Zoom Q8 makes a good companion by providing a wider shot or a two-shot while your GH4/VG30 is on close-up. The Zoom Q8 is actually cheaper than any balanced XLR solutions for GH4/VG30 ($2000 YAGH and $700 XLR-K2M respectively) and you get another camera angle as a bonus.
This extra angle will help you synch your audio and can be used to aid continuity by preventing jump cuts in single cam set-ups.
For example when shooting an interview, you can cover a 2 shot with the Zoom Q8 and cable up the interviewer's and interviewee's lavs into separate XLR inputs on the Zoom Q8. You can use your main cameras just for head shots. In post, you can pan the 2 audio tracks to centre balance and mute the silent party's track when just one person is talking. This will significantly reduce ambient noise compared to having the 'air-track' in the mix.