There are a few things to consider here.
First - to anyone in the future reading this wondering about similar situations, make sure to talk this over with your employer prior to this situation. Some companies have different policies on this. Some will consider any education received while working as a non-salary changer, some will adjust accordingly. The important thing is to know this in advance. For this particular situation, it's a bit late, but anyone else seeing this - try to determine this ahead of time.
Second - you should talk about this sooner rather than later. It sounds like you are being significantly underpaid compared to market rate for your position and someone with a 4-year degree is. You don't want to find out come January the company isn't going to be able to give you a raise or anything and suddenly be stuck in a low-paying job (well, maybe you do?). You could approach this subject like
- "Hi Boss, I was getting ready for graduation and something occurred to me - we never talked about how formally receiving my degree will affect my employment here. Will I receive any increase in responsibilities, salary, or is there anything else I should know about?"
- "Hi Boss, I will be graduating in a few months and will be able to take on more responsibility at work as a result. I was wondering if this will be reflected in my salary?"
- "Hi Boss, as you know we recently reviewed my performance for the first 3 months and it was very good. I've been able to contribute XXX and YYY in my time so far and will be graduating soon - I know that normal salaries for full-time people in my position are in the $65k - 75k range, is there any chance we would be able to reflect my degree change in my salary?"
Try not to approach things from a "I have a piece of paper pay me more now" perspective but a, "I'm going to be able to add more value than before, can I be compensated more" perspective.
Also read through the related links Shog posted as a comment.
Third - realize your piece of paper DOES make you more valuable. Some companies (admittedly less in the tech industry) care about that 4-year degree piece of paper. This has the effect of increasing your value on the market, as you are aware from your salary search.
If it is a difference of 25% then it is 25%. Just because you might have a job that is only paying 50% market rate right now does not mean you should only ask for a 10% raise. Or whatever the numbers might be.