A large number of developers that recently joined means 1 of 2 possibilities: rapid growth, or high turnover. Rapid growth is great for a business, but it also means that they’re figuring out new processes and assembling teams out of people who have never worked together before. It’s good to keep in mind when joining, to try to understand what challenges that brings.
High turnover can be a red flag. It could mean stressful work conditions where people rapidly burn out and quit, a high pressure environment where people are quickly fired, a company that’s on hard times, or a reorganization to save a project. To me it shouts “proceed with caution” but it doesn’t make me run for the door. When interviewing at a company that shows signs of high turnover, it’s even more important to ask a lot of questions and try to build a sense for why that’s the case.
This can act as a proxy for job satisfaction within developers. It’s not impossible to get a single developer to stay in a bad situation for years and years, but it’s much more difficult to find several on the same team. Four years is a very long tenure at a single company in startups, anywhere that has multiple developers who have worked there for two or more years has likely gotten their culture above a certain threshold.