When registering 4D products only one Master Product license can be used at any given time. If multiple licenses are registered on the machine, the application will pick a random license from the list upon launch. It is important to understand that the user is unable to choose which license will be used if multiple eligible licenses exist. This can be problematic since the features available to the application would change depending on which license is used, which could make troubleshooting an issue very difficult.
Let's look at 4D Server first. There are multiple licenses that provide access to 4D Server, such as 4D Server and 4D Team Developer as well as the Partner variant of each. All of these licenses provide access to 4D Server but each license has a very different subsets of features. If, for example, both a 4D Server and a 4D Team Developer licenses existed on the same machine, 4D would randomly choose one of the licenses present each time the application starts.
Now let's look at 4D single-user. There are multiple licenses that provide access to the 4D single-user environment, including 4D Developer Standard, 4D Developer Professional, 4D Web Application Server, and 4D SQL Desktop as well as some Partner Program variants. All of these licenses provide access to the 4D single-user however the feature sets available for each are very different. If multiple licenses that grant access to the single-user environment exist on the machine 4D will randomly select one at launch time. Subsequent restarts of the application may use a different license since it is randomly selected each time the application starts.
The situations listed above can be avoided by only registering one Master Product license per machine; i.e. one 4D Server and one 4D Single-user.