Borrowed from a comment I made to another answer asking a similar question:
There's known line-level issues with the H4n, so unfortunately it's not quite that simple (thanks to Samson's/Zoom's fault here). It's a Hi-Z (unbalanced, high-impedance) which still feeds thru the preamp instead of being a direct, true professional line level. Leads to distortion problems even with the input on the Zoom set at below 1.0 (newer firmware actually allows you to set it below 1.0 - it used to not be able to). Word is that an inline attenuator of 10-20dB is needed between an out-board preamp and the H4n line-in if you want to get the most robust gain stage.
I don't remember the link but do a Google search about this and you'll find a step by step for creating the correct signal chain to get a decent Line level signal through the H4n.
The problem with the H4n is that the line truly isn't Line at all - both Mic and "line" level are routed through the preamps, whereas true Line Level bypasses it. So any way you slice it, the signal has to ho through its pre's. Poor design in my opinion.
So that being said, your mixer is probably fine - just seek out a different recorder which actually offers true Line level throughput - there's many other recorders out there in the same general price point/style as the H4n but actually do offer true Line Level. If anything, they should all offer Line Level because it requires effectively zero cost to implement (so why this not the case on the h4n is beyond me) - its the pre's for mic level that the cost/engineering comes in for a recorder.
