The fact that you were a student is what qualified you to purchase Lightroom at the student discount price. Now that you have a license that you have purchased, you are eligible to purchase any future upgrades. There is no time limit on that, and you don't need to be a student in order to purchase the upgrade installation. You might want to consider using the DNG converter, and waiting just a little while to see if Lightroom 6 is released. Nobody knows who when the release date is, and historically it has been that those who purchase Lightroom within 30 days of a new upgrade will qualify for a free copy of the new release. But the choice is yours. In direct answer to your question, yes, you can purchase the upgrade. Just one note of caution. Make sure you keep your original serial number where you will have access to it. It is required in addition to the serial number for the upgrade version. And if you install an upgrade on new computer were Lightroom 4 isn't installed or hasn't been installed then you will be required to provide that serial number as well as the new one.
As Jim says, guard those serial numbers with your life; the best bet is to register them both with Adobe, and then they can't be lost, as seems to happen with so many careless people who then desperately ask for forum help.
Thank you very much,that was very helpful! Especially regarding the wait for LR 6. Didn't even think about that and since I don't yet have the Mark II, waiting is not an issue if I can use the upgrade.
I am guessing I will need the serial number that came from Adobe after the verification of my student status? Or do I need the serial number on the box that I bought? I still have both, but only the serial number from Adobe is safely put away in the cloud.
To be clear, you only need to qualify as a student to purchase your initial version of Lightroom. Once you have that serial number that begins with 1160, you qualify to purchase any and all upgrades in the future at the upgrade price. If Lightroom 4 is still installed on the computer, then the upgrade installer will find the Lightroom 4 serial number and you will only be required to provide the new serial number for the upgrade. If you decide to purchase a new computer, which will probably happen at some point in the future, then Lightroom 4 will not have been installed and you will need both the Lightroom 4 serial number as well as the serial number for the upgrade installation.
We see a lot of people coming to the forums and say that they no longer have their serial number and want to install Lightroom on a new computer. Serial numbers are critical, and should be recorded in multiple places. One of the best ways to make sure that you can always access your serial numbers is to register your software after it has been installed. Then you can always access your Adobe account and retrieve required serial numbers. But as a safeguard, it would be a good idea to store serial number files on multiple hard drives and even in a physical card file. Without the serial numbers, it's like having a new car and no key to start it.