4.x version of react-engine removes the dependency of react-dom from the project. Users of react-engine should install react-dom along side react going forward.
While upgrading to 3.x version of react-engine, make sure to follow the react-router's 2.x upgrade guide to upgrade react-router related code in your app. Then, add to your express error middleware, react-engine's MATCH_REDIRECT and MATCH_NOT_FOUND checks.
2.x version of react-engine brought in a major api change. Basically it affects the property names of the object that gets passed in during the engine creation on the server side and also how routes definition is passed into react-engine.
In v2.x, routes need to be explicitly required and passed in to the engine creation method. Also, any react-router known properties can be passed in.
An example engine creation can be found here.