When you are designing and architecting solutions based on SharePoint you need to be aware of the various limits and boundaries that exist either because of the underlying platform design or the physical and practical limits of the operating system, network and hardware. You can find a large number of these by reading the Capacity Management article on TechNet. I have tried to commit these to memory, but I always forget, or get them mixed up. Can I have 300 web applications per content database, or is it the other way round (yes, it is)? Hence this quick reference.
Most of these limits have been determined by building a test lab and load testing the test environment to measure performance at various levels of load. Sometimes performance drops quite suddenly at some threshold, but usually these limits represent a point at which the performance will start to gradually degrade. In some cases there is a "supported limit" which means that this is the extent to which it has been tested - beyond this the behaviour of the system is unknown and that is why it is unsupported - you are on your own.
Ultimately, most of these limits are guidelines, and are affected by other factors in your computing environment such as hardware scale and performance. If you really need 4 TB content databases, then build a test environment, preferably using production hardware. Check that you get adequate throughput, and that you are able to back it up in a reasonable time that meets your operational requirements.
I will endeavour to keep this information up-to-date and accurate as guidance changes. If you find an error, please contact me.