Applying for another PhD after dropping out 4 years ago, should I use my previous PhD supervisor or undergraduate supervisor for a reference?
Assuming your resume includes the fact that you spent 4 years doing PhD research and were not awarded the degree, I don't see the problem with asking your former supervisor for a reference. I assume this is the case, because you still need to account for the 4 years in your work history. In that case, your supervisor's letter could be valuable in explaining that you weren't told to leave the program due to other concerning issues; this was for health reasons. 

But here's the key: can your former supervisor write a strong letter of recommendation? Ask him or her explicitly whether they can do that. If not, move on to a different reference.

Think about any other references you might be able to call upon in your current position as an alternative. Do they need to be professors? Have you worked with research associates or other professionals who can vouch for you?