How do I find a good team that sets me up for the mid-level -> senior level promotion?

• You are correct in that you want a healthy mix of levels if you're targeting L4 (mid-level) -> L5 (senior) promo.
• Teams that are too senior (e.g. everyone is L5+) are bad as all the good scope is taken and you have nobody to mentor.
• Teams that are too junior (>80% are L4 and below) don't have anybody to mentor you and are just bound to implode in general as every good team needs a strong bench of L5+ engineers to survive.
• As an L4 engineer, you specifically need L5 engineers to mentor you, and L3/L4 engineers that you can mentor to show that you can work through others.
• The other thing to consider is the amount of scope a team has. In particular, you want to avoid a team where its product is too mature and optimized. On these teams, you're going to run 10 experiments a half to increase metrics, and it's likely all of them will fail as the product is already hyper-optimized. An example of an overly mature team like this would be the main feed in the Facebook app.

Related resources:

• "How do I come up with innovative, impactful ideas and bring them to my team?"
• "How to identify projects that are more suitable for senior engineers?"