I also refrigerate my peanut butter to keep the oil separating.
Any oil can go rancid (oxidize), and how fast it happens depends on the oil and how it's stored.
Peanut butter is a monosaturated faty acid, thus, is less likely to become oxidized than polyunsaturated fats, which are chemically simpler fatty acid chains, and include fats such as sunflower or safflower oils (and most of the fat in whole wheat flour). Saturated fats are the most complex of the fatty acids, thus, relatively less likely to go rancid.
Sun and heat increases the chemical reaction with oxygen, as does salt. Given this information, unsalted peanut butter will last longer than salted, and, unsalted peanut butter stored in a light-free environ will last even longer, relatively speaking.
If you want more information, check out this article:
http://findarticles.com/p/articles/m...93/ai_15714527
In my experience, 4 weeks on a tour is probably ok. If it gets to taste rancid (to me, the taste is bitter), it won't kill you, it will just taste nasty. Mold, however, is another matter.
P.S. Also, in my experience, warm/hot peanut butter leaks no matter what the storage container is. Wrap the container in a plastic bag (zip style best, if possible). You'll save yourself some washing, and then some. There's nothing more irritating than trying to wash peanut oil from clothes (and inside panniers) while touring.