As mentioned in the first chapter, one of the main differences between MongoDB 3.x and MongoDB 4.x is that the WiredTiger storage engine is now the default. Accordingly, you will need to ensure that you take advantage of WiredTiger's multithreaded capabilities by adjusting the RAM and CPU usage. There is an excellent guideline given here: https://docs.mongodb.com/manual/administration/production-notes/#allocate-sufficient-ram-and-cpu.
One of the main differences between installing MongoDB 3.x compared to MongoDB 4.x is the list of operating systems supported. When considering upgrading a server currently running MongoDB 3.x, consult the supported platforms guide (https://docs.mongodb.com/manual/installation/#supported-platforms) in the MongoDB documentation. Generally speaking, each new version of MongoDB removes support for older OS versions and adds support for new versions. Also, as a general consideration, where versions of MongoDB...