I’m sorry this is so difficult for you. It was wise of you to leave the house. Sharing space while the two of you sort this out would be painful.
I’m not convinced from what you wrote that your boyfriend is depressed. It’s possible, of course. But it’s also possible that he is trying out what it means to be on his own and is partying with the idea that he is reclaiming being single. He may be trying to meet new people rather than hang out with people who know you both and who have opinions about who he is and what he should do.
If he is depressed, there is nothing you can do to encourage him to get help that you haven’t already done. But if he is truly wanting to take a break from the relationship, the best thing you can do is take some distance for yourself.
What you didn’t mention in your letter is how you are doing. I imagine that the possibility of really losing him after 4 years is very difficult. I hope you are taking care of yourself and thinking about how to move forward if he decides it is truly over between you. Do reach out to friends and do whatever you need to do to grieve, to learn from the experience and to heal.
I wish you well