Occam's Razor states that the simplest solution to any problem tends to be the correct one. We only know 3 things pertaining to the actual status of 4959 Loader-Dozer as an unreleased set:
1. The set was the same as 4950 Loader-Dozer and was to be sold for the same price. (Even the "official" picture on the 4959 page is identical to 4950.)
2. The set was not advertised anywhere else except a single catalog.
3. 9 and 0 are immediately next to each other on a keyboard.
Given this evidence, would it not be a far simpler conclusion to make that the "unreleased" 4959 is really just 4950 with a typo? Is there any evidence elsewhere that Lego would re-release a set while the original was still in stores, or that they were planning to do so with 4959?
Yeah, it was only in a German catalog as far as I'm aware, which was laid out exactly the same as other catalogs except 4950 was replaced with 4959. I don't think it deserves its own entries in lists but unfortunately it's already on every site listing rock raider sets.