It's easy to upgrade - you just take the "old" ticket to the Guest Services window at one of the parks (or DD) and tell them what you want. If you just bought the 4-day ticket, you will just pay the difference between the 5 day and the 4 day.
However, Disney will only credit you the actual purchase price of the "old" ticket. So, if there has been a price increase since you purchased the "old" ticket, you will be credited the actual purchase price. In that case, you will pay the difference of the "old" 4 day price and the "new" price for the 5 day hopper. So you will need to consider the cost of just using the "old" ticket and paying for another day's admission if you need it. It's one of those times that you just have to "do the math", to see which is the more economical choice.
Hope that helped. Enjoy the trip!