If you really want to keep your Zoom, but want a portable, bulletproof preamp with gobs of very clean gain, you will not do any better than a Sound Devices Mixpre AKA Shure FP-24. I own one as you can see in my signature, and I am never selling it. These pop up on the YS here from time to time.
This one looks to still be for sale, and at a crazy low price.
If that doesn't work out, you can find these on eBay here and there, but be wary of condition and don't pay more than $300-$350.
The other way to solve your problem is to simply replace your H4n with a better recorder which has much higher-quality built-in preamps.
Tascam DR-100 mkIII is what you should get.
As much as I love my FP-24, if I was in your situation I would definitely replace the H4n with the DR-100. Besides better preamps, you are gaining better ADC and the ability to record a pair of safety tracks at lower volume so you don't need to run limiters if levels go too hot. It's all-around a much better recorder, and you won't need any additional cables or need to carry another box, extra batteries, etc.