Leaving a job that you truly benefited from and grew in can be a very bittersweet moment. I think you're focusing too much on the impact and purpose the job has, and not enough on the growth you've experienced; I'd posit this is what makes the experience truly emotional. I had the same emotional reaction after leaving a restaurant job I worked 6 years because it's where I grew into an adult and built a work ethic that will suit me well in the future. Realizing these kinds of impacts can be really emotional.

The good news is, growth is possible at your new job too. You can leave there after 4 years and feel just the same way, it just really depends on what you do with your time there as well.