As someone who has dealt with depression far longer than 4 years, I'm hesitant to even respond to this. I don't want my experience to discourage you. Just because it's been an almost lifelong struggle for me doesn't mean it will be for you.
I had some supportive family members and a small group of very good friends who played a big part but the main thing that sustained me was my dream/goal. I wanted to at least accomplish a part of it. I did. I didn't accomplish what I wanted to accomplish but I got fairly close at times. I lost huge chunks out of my life but then there were also times when I could at least get a taste of fulfilling my dream.
My biggest regret now is not trying harder. More specifically of not being willing to risk the humiliation of a public breakdown.
As for suicidal thoughts... I guess that the positive things mentioned above helped some. Also the fear of death. The question of whether my death would have a net positive effect on my family or more negative effect was also an issue.
Sometimes it was sheer stubbornness. Refusing to quit... refusing to accept defeat.
Try to keep one simple but often overlooked thing in mind. In addition to being the worst enemy you will ever face, depression also sneaks into your camp and tries to recruit you to its side.
I don't know if any of this will help. I suspect not. I think that in the end we all find our own way. I hope you find yours.