The possible new Davis Cup format that will start next year and that will be announced in detail in August, is still "work in progress." The International Tennis Federation (ITF) will keep reuniting several times until 4th August, when the whole Committee will vote for the definitive format.
There are some important news according to L'Equipe. From the last meetings, the French newspaper understood that the four singles matches will be played at the best-of-three sets on Friday and Sunday, and the doubles match will be played on Saturday as best-of-five sets.
So at the moment we don't expext to see Davis Cup ties being played only on Friday and Saturday. Among the many cards on the table, 'Final Four' format, it means playing both Davis and Fed Cup semi-finals and finals matches at neutral venue, is being considered as well as the possibility of having round robin.