Who should I ask for references when applying for PhD, after a 4 year break since graduation?
I agree with the other answers that you should ask old professors. I was in your situation after a break of five years, and while I felt awkward approaching people I hadn't seen in so long asking for a favor, the particular professor I asked actually did remember me and was happy to help.

I'll reiterative the general advice I was given (by a current friend who is a professor in another field, both of us in the US):


Try to get at least one recommendation from a professor who's had you as a student. Not having an academic reference would be a disadvantage, and having a recommendation that speaks to your ability to do research is an advantage. 
Recommendations from industry are fine, normally your first choice would be an immediate supervisor. Your approach will depend entirely on your relationship with your peers. 
Recommendations from friends typically aren't great, but if they're in the same industry as you, have collaborated with you on stuff, and can speak to your professional competence then they're probably good.