Same problem here. I asked paradox interactive and they said this causes AMD driver.
The game crashes every few minutes.
imac late 2015, 3.2GHz i5, 8gb, AMD Radeon R9 M390 2 GB
Anyone on iMac getting "hoi4 quit unexpectedly" should try turning off Shadows, Weather, Reflections, 3D Buildings, and 3D Units. Paradox told me it's a known issue that AMD needs to address. Their fix had me turning off everything but sync and rivers, but it looked terrible. Luckily I was able to get it to run with these settings...I actually prefer it without the 3D units.