what electrical cable to use

I am looking to run power out to a camper on my property (from the house). I'm going to need about 400' of wire, and may use up to 1200-1500W of power in the camper at the most. I want to be safe, and I want to minimize the amount of voltage loss, though balancing that (volt loss, not safety) with my limited budget.

I am currently looking at 4 (or 6) AWG THHN Building Wire Edit:moderator (link removed.) The wire will be outdoors, of course, and I probably would like to bury at least part of it -- the burying is not essential, could be foregone if problematic. First off, do you think this type of wire will work well for the purpose I have described? If so, do you think the 6 AWG would suffice (how much volt loss would you anticipate)? I have seen a few different calculators for this online, but their final numbers significantly vary.

I am open to hearing any suggestions on what would be my best choice for running power out to this space on the cheap, including good sites to visit.

Thank you for your time!