Break ups are hard. It’s cliché but time will heal the wounds but it’s what to do in the mean time that is the hardest.
If your ex is already dating it could be that he’s just venting. He really could just be occupying his time. The flip is he could have already been dating someone else and that’s what all your girlfriends will tell you to make you feel better but it doesnt. It doesn’t matter if he was or not what hurts is he’s seeming to be doing just fine and you’re not.
In breakups hurt and pain comes in waves. The only wave you see is that of your own. You’re not responsible for his way of getting over you. After 4 years and time invested you are crushed. My advice would be find you again. We all lose a part of ourselves even in the most balanced relationship. If you feel like going out and hanging with friends do so. Don’t feel like you have to look or come off a certain way.
You do what you feel is best that is safe and non harming of course. Make sure that you enjoy or find some me time activities. Eventually you will get back to where you need to be and it may not look like the time before your boyfriend. Heal. Don’t rush into anything I would put off hooking up with someone too. You don’t need the company and the headache that comes with a rebound. Try and again I say try not to be over consumed with how he’s doing. If need be take a full break by cutting off access to see his social media pages trust me all you will do is worry yourself. Good luck to you.