Now I have a question regarding cap space accrual - Let´s say Team A trades Player A with a cap hit of 4M, halfway through the season for Player B from Team B who also has a 4M cap hit. Do both teams reduce their cap hit by 2m? since 50% of that player is already paid for and they loose their 4M cap hit of the player who started the season with them?

And teams that don´t do any moves stay at the same cap hit that they had at the beginning of the season?

They would carry the same cap hit, unless one of the teams retains salary.
While technically organizations manage the cap daily throughout the season, that's not really how it's viewed by laymen (nor would we really need to).