Title says it all, really: does HoI4 has the same timeline of HoI3, or has it been modified?
The timeline of Hearts of Iron IV version 1.0, is from 1 January, 1936 – indefinite.
There is no technical end date at the moment, but content like researchable technologies are only featured up to 1948.
There is also a 14 August, 1939 starting date.
There are currently reports of players who have played to the 1960s and fought out "WW3" - i.e. subsequent High World Tension-triggered wars, for instance between the Allies and the Comintern, following the defeat of the Fascist faction in WW2.
Note that there have been reports that the very late-game can be very taxing on computer resources, as of version 1.0. This may make the extremely late-game impossible to carry out in practice.
You can start the game in either 1936 or 1939.
You will be shown a score screen after 1948 if there are currently no wars involving 3 or more major nations. You will then have the option to continue playing. (Achievements are still available if playing a 1936 game in Ironman mode.)
See this forum post for the exact definition.