Men covering their heads

Assalaamoalaikum Mufti Saheb

1) Do men have to wear a topee all the time or only for Salaat?

2) If a person is driving and it is very hot is he allowed to remove the topee because of the heat and he has no Aircon in the car?

3) Can we perform Salaat behind an Imaam who does not cover his head?

4) If a man wears a scarf to cover his head, is he fulfilling a Sunnat or does this scarf need to be tied into a turban to get the reward of a Sunnat?

Jazakallah khair