Thinking about grad school for computer science. How to talk to my old professors now?
If you are in a position to do so, go visit in person. Tell them what you've been doing and what you want to do. Give them a good picture of you as a success story. Catch up a bit on old times and what is going on with the newer students. Thank them for what they've already done for you. 

All of that is worth doing even if you don't want letters, of course. 

Doing it in person will help jog their memories of you as email won't. They've had quite a few students since you left and those are more likely to be remembered than someone from four years ago. 

But you probably want to do most of that anyway even if you have to do it by mail/email. You might need to be a bit more complete about what you've been doing since graduation, describing projects and such. 

But in general, professors like to hear from previous students, and, in general, like to help with their careers. They might even appreciate some guidance from you on what to stress in any letters. 

They will also, likely, be full of advice about where to study and how to go about applying. 

I would probably give the same advice no matter what the field of study. 

Note that you will probably want recommendation letters from current or recent supervisors at work, as well.