When you bought LR4 you got a serial number for it. All you need is that serial number to load a LR5 upgrade. When you first open LR5, after you install it, it will ask for the license/serial number and since it is an Upgrade serial number it will then ask for a serial number for an older version of LR. At that you enter the serial number for your LR4 and you are good to go.
I bought LR4 through the mac app store. One click, download and auto-install. I don't have any serial number for Lr4.
Ok then read this article.
It clearly states,
In a blog post on Adobe's website, the company noted that upgrade pricing for future versions of Lightroom may not be available for those purchasing the Mac App Store version, due to Apple's current lack of support for such a system; additionally, users may receive updates later than they would through Adobe.
So I guess you are going to have to pony up the full $149 price for Lightroom 5.
You could try going to Help>Lightroom Registration click the next button, see if there is a serial number entered (it will be stars * ) then Go head and register. Once registered the serial number should show up on Adobe's website once you have logged in with your Adobe ID.
Also if you search for a file ending with a file extension of .lrreg that should turn up the serial number. But since it was bought through the Mac App Store it might be a generic seial number that isn't good for upgrade pricing.
If you log into the Mac App Store with the same Apple ID that was used to purchase Lr4, you should be able to find and download the app under "Purchased" ... which should then allow for the Lr5 upgrade installation to work.