How to choose EC2 instance type for reverse proxy (4 million requests / month)

Any advice ?

Don't guess it, benchmark it and then adjust it. It's easy to switch between instance sizes in AWS so start with one that may be the right fit and observe how is it doing. Then increase or reduce the size to optimise your cost/performance.

By the way 2 requests per second is not much, almost any instance should do. The problem is that it's probably not a uniform load, instead it will likely be going up and down and in peak times it could be a lot more. You may eventually want to have a look at auto scaling, but it may be an overkill.

Your usecase can also be handled by an Application Load Balancer - you can create rules to direct the traffic to different backends based on the URL path. It can also handle SSL certificates for you, etc.

Hope that helps :)