You can definitely upgrade the parkhopper. You will receive credit for the amount you "paid" for the 3-day pass, which I believe would be $149 because that is what they are selling the advance passes for. You would be charged the difference between that and whatever price they are selling the 4-day passes for at the time you upgrade. I did this last summer without any problem; upgraded to an annual pass for $70; I was given $139 credit toward the $209 AP (went up to $229 the next day).
If you call guest services, they should tell you that you can upgrade, but they are somewhat clueless about how much it will cost. I was given different information than what was actually true, which is in line with the approximately 50% accuracy rate of Disney CM's.