The H4N, while a solid little device, is notorious for having weak on-board preamps. It works great in loud situations, where less gain is needed, but doesn't have the power for quiet/high-sensitivity work.
I looked at the link you posted. Haven't heard of this product before, but it's an interesting little device. The only thing that concerns me, is that it is a fixed boost of 20dB. I think you'd be better off saving up the cash to buy something like a Sound Devices MixPre. Not cheap, but nowhere near as expensive as some of their other gear. It would also give you two channels, so you could feed both inputs of your H4N if you add another mic in the future.
Another option would be to look for a used field mixer on eBay or some other place. You can get a used Shure FP31 very cheaply now-a-days, though it is only a mono output. Shure FP32 and FP33 are both 2 channels out, and you might be able to find one of those for a reasonable price.
P.S. I seem to remember something about the H4N's inputs being Hi-Z at line level, but I'm not sure. You can try contacting Varun Nair. I know he found something about the Hi-Z issue that prompted him to build some in-line pads to deal with it. Might be in your interest to find out what he knows.