Why still many companies use Qt 4.8.7?
Why still many companies prefer to use Qt 4.8.7 instead of choosing Qt 5.x?
Are there any advantages with respect to license, source code, portability?
The main reason is that it is (can be) costly to port to a new version.
Some things might need to be implemented in another way as some classes are gone.
There is not any benefits to use older versions except to keep old code working.
Old version are not free or in any way different in regards to licence.
To add to @mrjj, some of these project went through a certification process that takes time and money and just updating for the sake of updating would mean going through all that process again.