Also note that you haven't taken into account the possibility of drops. Your first calculation, for example, is just the probability that you get at least one boost in four turns. It does not mean you have a 46% chance of having +2 evasion at the end of those four turns, as you could get some evasion drops, too. It's entirely possible that you get one boost and three drops, leaving you at -1 evasion after four turns. The actual probability of having evasion > 0 at the end of four turns will be lower than 46%.
It's also somewhat tricky because once you get +6 you can't get another boost, so that slightly affects the calculation somewhat, but probably not so much that 46% is too far off from at least one boost in four turns.